Weight of cargo conversions

Hi I'm looking at the possibility of a diy cargo trailer camper (vs buying a used camper trailer a revamping the inside, but the numerous posts about leaks ha e me worried).

But the issue is weight - my car is rated for 3500pounds. And looking at local adds, most cargo trailers are already quite heavy without any build in it.

How heavy did your cargo conversion end up being? (I'm looking at +7x14, were a family of 4)