Yeh I just adapt the lap seatbelt from the centre position of the rear car seat of older model cars.
These are the lift/tilt the buckle to adjust, lap type belt.
Newer cars have "retracta reel" over the shoulder belts for this position and aren't really suitable.
Car wreckers are not allowed to sell seatbelts anymore, (farmers would buy the buckles to plug into the stalks, so the utes/cars alarms or beepers don't keep going off while in and out of the utes fencing checking stock etc.) so I hit up my mates that race speedway and always wrecking cars for part.
They are attached to my seat mountings with 8mm bolts.
I have had some bad slaps onto rock hard salt and the seatbelts have saved me every time. Had some bad bruising around the hips where the belt caught me though.
It would have been really nasty without it. One crash I was hit by a gust and flipped sideways over the mast onto the frontwheel then my seat back, frontwheel and seat back once again. That really hurt where I fell onto the sheeted in boom each tumble.
I struggle to wonder how they race without them.