The clamps used to hold exhaust pipes together is what I used for a similar trailer (I sold it, so I think I`we got no pix). You need to put something more in there. I used two small L brackets, made for holding wood in place (corners and so on).

Also, depending on the size of your seatpost you could chop an ahead stem and atatch to the seatpost and then DIY from there. I used one to put the twist grip shifter for my detatchable bike.



This setup has got a short piece of bar innside, just enough to put the shifter. You must think f a way to atatch the trailer but it can be done.

For the bed: plywood stickng out sounds good. also if mounts for wheels could be mowed forward, but that is going to cost. Maybe some "new" bolt on atatchment for wheels?