I built mine this time for stability. I ran angle iron from home depot from the rear wheel axle to the trailer bed and bolted it there. Trailer wheels are 16 bicycle wheels supported on both sides by the same angle iron. then a plywood bed to make it pretty much any length and width you want,.
It makes the bike very stable but the trailer has no swing so it is harder to steer. Not sure you want to build it this way.
I'm wondering how your bike trailer worked out, as we're starting a trailer ourselves also with 20" wheels. We're having a problem with the axle though, any ideas? 3/8" fine threaded bar isn't going to work with the 'bicycle' thread fasteners and bearings. Not sure if we can make a mount bracket for the outside end of the axles, we were thinking of one running the width of the trailer plus wheels.