I cant say ive built a case with perspex but i can give you some general pointers... First of all you are going to need the measurements for the slots, Specks can for an atx case can be found online or i wouldnt be suprised if someone in these forums had them. The second thing i can tell you you may very well already know, Your two most important tools are a pencil and paper. Plan everything to the smallest detail, others may disagree but i would go as far as to cut out cardboard or some other disposable material construct a test. The structure wont hold any thing(duh :P ) but you can check if everything lines up like you want it to and you have enough cable reach for your devices. As far as actually binding the case together, you might think about a clear AB Epoxy resin ive had a lot of success with it. It dries clear so you wouldnt see unsightly screws or anythign of the sort. but again i have never used perspex so i cant say how it would work.