I know most smaller trailer builders will make these. Gatormade did make one. There are the obvious list of pros and cons to these trailers. Hank trailers in middle TN will build one.
My only input would be to have it made with 2' sides and with 5K lb axles. With this it will hold alot of mulch and can haul a small miniX or small SS.
Knowledge is knowing a tomato is a fruit, wisdom is not using it in a fruit salad.