Your insurance agent is delusional if they only took the seats and belts.

If you want to continue to use your rear seating area as a place for people to ride, I would strongly encourage you to replace the stolen seats with another set of stock seats.

Otherwise, there are plenty of fixed back lightweight seats that will work for the DC2 chassis. I would recommend the Sparco Rev or Sprint (I had a set of the latter in my GTi)... along with camlock harnesses and a bolt in harness bar for the shoulder straps.