If I understand correctly you're thinking of building a small version of a Lance or similar type of pick-up bed style camper specifically to fit your existing trailer?
FWIW, It seems like a mini truck 4Wheel Camper would fit that trailer. Then you wouldn't have to build something, just get one and go.
I know, sometimes the journey (building it) is far more important than the destination (using it).
My first thot echos Grim's re: duplicate structure. I think that can be mitigated to a large extent with some thot. If the trailer sides can handle the loads, then the camper structure inside the trailer need only be strong enough to support the unit when it is not loaded into the trailer.
I used to swerve around my hallucinations, now I drive right through them.