When I built my Wraith, back when dinosaurs roam the Earth and 3D printing was beyond the reach of us mere mortals, I used some velcro straps cut down to about 1/8" wide as a lap belt to keep the driver and passenger in their seats. Now that I can print TPU, the velcro just had to go. These are designed specifically for the Axial Wraith with Chell (from Portal and Portal 2) and Sam Fisher (from Splinter Cell) sitting in the original seats, but I suspect they'll fit any other 1/10 scale seats and figures.

The shoulder belts go through the holes in the seat backs, then cross and go down to the screws that hold the seat back to bottom. Lap belts go directly to those screws. The anti-submarine belt is kind of a pain, but it's just trapped between the seat and floor. It's a total pain to get in the right place, and is purely aesthetic, so I just cut it off. If they're too long or short, scale them to fit. The screw holes will conform, because TPU.