Hello Johnny,
the weight depends on the material You are using.
Remembering my first load on a bicycle) trailer with about 130 lbs , I reduced the load weight to 60 lbs by building it with 20 mm Airex foam and 2mm plywood not coated with GRP.
So I am not shure what speed You are using to drive,but I think 20mm Airex 80 (kg/m3) is strong enough and You have a good isolation and no stalactive cave…hi
regards
Knut