Take a look at my thread of my build (http://rib.net/forum/showthread.php?...=super+student
) ...somewhere in there you will find details of how I made my console by glassing over ply. I was using 9mm and 5.5mm ply and joined most of the corners by filleting with epoxy. With some of the right angle joins I used 1x1inch baton, stainless screws and that expanding wood glue stuff...its not coming apart in a hurry
For what you want to do, the 18mm ply will be pretty strong and if you make the join between the top and the sides by having the top on top of the sides (join underneath the bottom surface of the top...does that make sense?!!) then there will be minimal load on your joint. A carefully wood glued edge should suffice with the glass over the top as well.
When you glass over the ply, you will need to use the stuff I specify in my thread (I think its called G4) to help the polyester resin stick to the wood (unless you're going to spend lots of money and use epoxy resin to wet out your fibre glass). An angle grinder is very useful too for rounding edges and taking off cured bits of glass.
Hope that helps a bit, I can go into more detail if needed.