Perspex is very tough, I used 3mm for the front of my case, 2mm for the rest. 3mm is very rigid; destroyed my chuck on my dremel (had to buy a new one).
If you relocate the PSone contrast and volume buttons you can fill in that gap in your case nicely. If you also fill in the holes around the buttons and paint the system you should have a nice looking system. Use regular paint (wall "feature" paint) on a mini roller - I made the mistake of using one-coat paint on my case front and it was VERY hard to work with.
It looks like your case top (screen, etc) is thicker and slightly bigger than mine (incorporating my N64 system)! Looking at the pic, it looks like the Wii mobo will just fit in my multi-project system, when I can get a Wii cheaply that is!





