We just got our Discovery and upon putting in my daughter's (4' & 50 lbs) booster seat - Nautilus 65 - we barely can access the seat belt "tongue" (part connected to the bottom of the seats). It's enough that we're having to look into new seats; ones with a narrower base.

What is everyone else using?

 

Cosco Topside Backless Booster Car Seat
About $15. Works well. Cheap and easy