Perspex stack style cases

Hi Folks,
I'm finally starting to design a case for my B7971 clock and twitter interface ( hackaday.io/project/9155-nixiebot twitter.com/NixieBot )

I'm currently going with a pyramid affair made of stacked lasercut perspex, probably mostly black with some orange layers for internal lighting to show through... see attachment.

I'm thinking of bolting it together using M3 bolts, does anyone here know what's a good minimum distance to keep the holes from the edge of the perspex (5mm thick) to prevent cracking?

Has anyone any handy hints and tips for designing with lasercut perspex? It'll be my first attempt at such a thing so any knowledge is good

Cheers,
Robin.