i'd have to say nix the idea about having lexan between your cp and monitor so you can see the motherboard. most people want a small arcade machine, aka small footprint, and this would greatly increase the depth. also, you'd have to design a cab to account for this extra room to view the motherboard, which to me, would look ugly do to the deepness. lastly, is it really worth all that work just to view the motherboard? might as well just leave the front of the cab open if you really want to see it, stick the motherboard vertical and just don't have a coindoor and have the lexan on front. but, to each his own (like i said before).
...or you could get some thick lexan and use that instead of wood for your cp, and mount your buttons to that and be able to look through and see all the stuff inside...eh, it just doesn't sound worth the money or time or space. good luck though, love to see how this all turns out.