Cage building question

[reptile boi]

Hey,
I was wondering, what size (thickness) of wood do you gise use, how do u assemble the top peice, and do you use any kind of special holders (i forgot what those metal thingys are called, but you use them to hold a peice of wood on top) in other words, how do you make a 5' x 3' x 2' enclosure stackable without the middle top part kinda curve/cave/collapse (whatever you wanna call it) downwards because at school the book shelves tend to do that and looks ugly and it looks like it might break anyday! And what do you gise use to add more strength to the enclosure?



Thanks,
Ben