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In the past i found that using a center punch, drilling a small pilot hole with a regular drill bit and then using a step drill to the desired size yielded the best results.

For smaller than the 1032L/1590BX i generally look through the hammond pages until i find something with the right dimensions.
http://www.hammondmfg.com/dwg.htm
http://www.hammondmfg.com/1550.htm

For anything bigger they do these: http://www.hammondmfg.com/dwg16.htm

I think generally anything bigger than a 1032L is a custom job at the sheet metal formers which unfortunately isn't going to be cost effective for a single enclosure

Thank you Komigenie, i gave it 3 passes with the laser to be sure, successfully etched and then scrubbed the unnecessary paint off with some wire wool.

Also thanks to Cody for the excellent tutorials 

I can confirm that the TLP222G can completely mute the signal when used in conjunction with relay switching as described on the stompville site.

My effect on code is as below, i set the "MUTEONDELAY" & "MUTEOFFDELAY" variables to 5000ms, when clicking the switch the led turned on, signal muted, a 5 second delay, hear the relay switch, another 5 second delay and then the guitar signal was unmuted.

{
  digitalWrite(LED, LOW);   // turn LED on
  digitalWrite(MUTE, HIGH); // enable optomute
  delay(MUTEONDELAY); // pre switch mute
  digitalWrite(RELAY1, HIGH);  // switch relay to on position
  delay(RELAYDELAY);  // wait for relay to switch
  digitalWrite(RELAY1, LOW);  // turn coil off
  delay(MUTEOFFDELAY); // post switch mute
  digitalWrite(MUTE, LOW); // disable optomute
  state = ON;
}

Edit: I'll also add that as this is a single attiny, its all on a single ground plane.

I think there might be more colours to come, especially if the ones they currently offer fly off the shelves.

Well that is a coincidence, lime green has just appeared!

I've got a stack of the white 1590BBs and so far they've all been exceptional 
I bought a single blue one and thats also really good, a nicer blue than the hammond pre coated enclosures (IMO) and i'm hoping they engrave better than the hammond ones.

I think there might be more colours to come, especially if the ones they currently offer fly off the shelves.

None of those will fully mute your signal in that arrangement.

Use a relay in conjunction with the opto to stop the pop but still fully ground the signal.

See the arrangements that use an opto and switch or opto and relay for bypassing for arrangement of the opto and relay in your setup.

Reading between the lines i think this is what is going on but he doesn't want to give away the full schematic which is fair enough as its for the BOTY.

Is the schematic for the relay & muting portion as shown here? : http://stompville.co.uk/?p=423 (about half way down the page)

OK, if you're using the I2C bus then yes keep AGND & DGND seperate and if neccessary join them only at 1 place, i tend to put 2 pads next to each other so i can easily pop a jumper on if needs be, i'd also suggest adding plenty of filtering in close proximity to try and keep the noise levels down.

The 222G appears to fully mute the signal when switching as i don't get any pops and the switch is more noticeable as there is a percieved silence, i'll confirm this tomorrow by editing my code and putting a much larger delay in.

When using the 222g to mute the signal I've always connected digital and audio grounds. The only time I've kept them separate is when using the I2C bus on larger projects.

I also enable the optofet directly from the microcontroller, I originally had a resistor in series to soften the mute but found it didn't make a great deal of difference.

Is the sleeve connected to GND at all?

Looks like a fun project, what are the approximate travels on the z axis? It looks like you have enough distance to set a pallet system up so you can hold a handful of enclosures at a time with multiple pallets for different sized enclosures.

Do you know what rpm the spindle is? i imagine it'll be good for engraving enclosures.

Be prepared to break end mills until you get the speeds & feeds figured out for what you're machining!

Well, i had a pop at this over the weekend and wasn't very successful.

I sprayed the FR4, let it dry, lasered away all the paint to be etched, popped it in the etchent(Ferric Chloride - MG Chems 415) and nothing... it would appear that enough residue was left behind to stop the process, next time i'll try 2 passes with the laser, it looked as if all the paint had been removed. I probably left it in the etchent for 45mins in the end. I then struggled to remove the paint that was left by hand, probably need to etch it and then run it under the laser to expose all the copper for soldering.

I then tried the usual toner transfer method for the first time, possibly a little too over enthusiastic as i tried to do a 150mm x 100mm board with 16 small pcbs on all at once, after a long soak some have transferred ok, i think i'll wirewool the lot and split it up into smaller pieces, rough it up with some 600-800 grit and try again.

I'm hoping i can get the laser method dialled in as its a lot easier & quicker!

Thanks for the heads up, fortunately I'm limited for choice for products like this and Ferric Chloride is one of the only products available off the shelf. For the rest of the process i'll use selfdestroyer's excellent tutorials, might attempt a reverse etch on some old enclosures once I've got enough used etchant 

Here is my best one, but its just a test rig:

Nice, is that a shared osh park project?

Ooo impressive, glad I'm not barking up the wrong tree for once

Time to buy some chemicals and tell the mrs i'm getting my Heisenberg on in the garage!

I want to have a go at etching a few PCBs, I've never done it before & me being me, probably want to make it more difficult for myself using the equipment I have.

If I were to spray paint a sheet of FR4 and laser engrave the etch artwork (I'll need to invert this so it removes the correct material) that should work right? I might need to try a few different types of paint to see what works best, and then obviously proceed with the normal etching method.

Has anyone with a laser attempted this?