I made my trailer from the chassis of a pop-up camper; the kind you crank up the top half, then fold out the beds on each end. I demolished the camper, painted the chassis, installed lights and wiring, and more recently put on new tires and had the wheel bearings serviced.
The bed is a full 4' x 8' sheet of 3/4" pressure-treated plywood with wheel chocks made of P.T. 2" x 3" lumber on edge, lag-bolted from below, and four eye-bolts for each of three possible bike positions, one centered or two side-by-side, although two bikes is kinda tight.