Here's my conversion. It is based off of a Burley flatbed as I liked the design and it seemed like an easy transition from the original trailer which was a Burley D'Light.

I stripped off the fabric and the high side rails. For the platform I used steel framing studs and for the new side rails I used 1/2" conduit. Both of which are galvanized steel. Using steel did add a bit more weight. The original trailer weighed 15 pounds, and I estimate it weighs 35 pounds in its current state.

I just went for a test ride with about 120 pounds. The ride felt very rubber band like at slower speeds. Is this common?

I'll cross post this in the trailer thread.