Well, i just used some plywood our neighbor tossed out some time ago. So far I have $30 in the trailer. Next it will get some weather proof paint, reflective tape, mount the flag and possibly add a small lockable tool box on the front.
click to enlarge.
EDIT: All done now. Added the tail lights and some reflective tape.
Kept it simple and did a 10 mile grocery run. Can barely tell it is there when going straight, more when riding up a hill but as stable as can be.