The idea was that the pan would slide out from the front. The aluminum structure is screwed down to form the table top / floor boards (depending on which way you look at it).
I've since abandoned the washing machine pan because it's too flat-bottomed. I would use or make something more tub like in shape for drainage.
These are 32"x32" exterior dimensions. Bad size because it's easy to find boxes and pans and such that are 24" but 30-32 is harder to find.