You might be able to have someone add a strip of "E-track"; and then add belts snapped into that when needed?
I converted a bare Transit cargo van. My design has the benches facing each other along the back, and I added aftermarket seatbelts by attaching them to the existing cargo tiedown points using chain-repair links. But having seen the insides of a bare-bones van, there are TONS of reinforced points that can be utilized - you just need to know where they are.
Ed, Deb, and 2 dogs
Looking for a small Class C!