Metal-top stick questions

I’m building (another) stick these days. This one has a metal top, however, and that’s something I haven’t dealt with, yet. I’ll be bottom mounting, so I guess any kind of taper I can give the screw holes, however slight, will help me fit the artwork/plexi over the screws. Is there any kind of a trick there, or just deal with the fact that the screws will be sticking up slightly? Yes, I know it’s only the height of the screw’s head, but it doesn’t hurt to be overly cautious. =)

What kind of metal would be best? I have access to a small range of metals. Anything to avoid?

What’s the ideal thickness for the metal sheet? …or does it matter as long as my metal + plexi == 8-9mm total?