If your proposed trailer tub will fit a M416 frame, then I may be interested in buying one sooner rather than later.
There are a lot of M416, and similar variants, trailers out there. Mine, for example, is from 1949. While it's in great shape, it's now in need of significant metal work. There are many others in my situation.
Sierra trailers offers a replacement metal tub for ~$1,500, and to the best of my knowledge, that's the only option available.
So there is some form of existing market looking for a cheaper bolt-on alternative, but the dimensions would have to be similar. From the pictures you posted, your tub height looks short.
When I was shopping for a trailer years ago, I came across every variant of the M416 you could imagine. There was one civilian version that had a shorter bed height compared to the standard M416. Your trailer looks identical to that one. If you find the time, can you post your bed dimensions compared to a M416? Here's a good drawing with M416 specs: http://forum.ih8mud.com/trailer-tech...ons-specs.html