are you using epoxy or poly resin? what cloth weights?are you making one time molds or using gelcoat and micro bubbles to stabilize the mold?i built aircraft molds and prototype parts for a few years,feel free to run any questions you may have by me.
the best thing for wheeling might be to just make skins and pop rivet them on.also if you leave a strip of foam(maybe 2"x1" cross section) around the wheel opening on the inside and glass it in it will add a lot of stiffness.composite airplane wings and surf boards are built on that principle.
if you don't want them to sag in the sun later you should precure them by wrapping them in black plastic and setting them on some hot asphalt while in the mold.they can reach 170* in the sun (in california anyway)
polyester resin shrinks real bad so you might make them 1/4 inch or so too big if thats what you are using