I used 2 x 2 legs, 1 x 4 frames ,1 x 2 leg braces, and 1/2" plywood tops but be sure you get at least 4-ply 1/2" plywood. A lot of stores sell 15/32" plywood as half inch and it is only 3 ply. This is plenty strong enough if you are going to have several of them screwed together.
I get on top of mine all the time and it is strong enough.