Well first I would ask what subs are they, what kind of sound are you going for, what kind of vehicle is it going in, what kind of music do you tend to listen to the most, how much power are you going to give the subs and etc.? :huh:
As for using 1/2" mdf typically you don't want to use anything smaller then 3/4" because mdf tends to split sometimes whether you drill it out or not. Also the resistive properties as it relates to sub use is much less then 3/4" If you could I would take it back and get 3/4" if not we can figure out something else. Answer the questions above and I could give you a better idea of what to build.