My latest trailer is made using a chain link fence gate with 2 aluminum bars running length and bolted to the front & rear of the frame. The gate frame and the bars each have a wing bracket bolted to it, with the threaded parts of the wheels being bolted to each bracket. I extended the hitch arm a couple feet to better accommodate longer pieces (up to 10ft boards comfortably, 12ft boards possible) & slipped a square steel pipe over it and bolted everything together In an attempt to make it stronger. So far so good!
My rear wheel houses the motor & has larger threaded pieces to bolt the wheel in place than a normal bicycle, so I couldn't use the typical hitch attachment for a reach around hitch, so I tried a suggestion made on Reddit to take a 4" caster wheel (make sure wheel is removable & has plate, not stem), remove the wheel, make a metal plate with matching holes & bolt the caster wheel base to the frame of the bike. Then, the tip of the hitch arm goes where the caster wheel was & then I hitch pin slips through caster base & hitch arm. I believe I have pictures of everything in case I'm not making any sense lol. I've been alternating my wheels between 20", 26" & 700c, but regardless this trailer can hold some serious weight, and even some full sized lumber. Attaching the wheels with my limited skill set & (at the time) limited tools at my disposal, was easily my biggest challenge. The wing brackets tied it all together. enter image description here