Waaay back in '74 I built my own trailer. Regular trailer wheels, light plywood box covered with fiberglass.
Semi streamlined box with a curved lid. Widest point of the axle is the width of the bike's mirrors so it will fit
through any space that the bike will go through.
Fully loaded it weighs about 250 pounds. No trouble at all. I've pulled it thousands of miles behind by BMW R90/6
up to 70 miles per hour.
I got this idea to build another trailer to haul two bicycles behind the BMW. Just a light weight frame with
tie downs for the bicycles. I doubt that the whole thing weighed more than 75 pounds with the bicycles on it.
Damn! I could not get that thing above 55 mph on the highway! The wind drag on all that round tubing on
both the trailer and the bicycles was tremendous!! It was like trying to pull a parachute!!
I eventually cut it up for scrap.
I still have the original cargo trailer.