a long time ago my ex BIL and i made one of these......we used the 3/8 for the end walls and half inch for the floor.........if i had to do it all over again, id make the walls with half inch. reason being is that the 3/8 seemed kinda flimsy when setting up and in use.......even with 4 wall supports in it, it buckled a lot with just a moderate breeze.
another this i would suggest doing is putting the floor supports at 16" o.c..........2 foot centers is just a bit too much for 1/2 plywood........if ya went to 3/4 for the floor, then ya could get away with 2' o.c. without any sagging when its under a load ( i.e. a couple of guys in there fishing, and all their equipment too)