I installed a temporary belt myself for the grandkids. Just to discourage the pre-schoolers from jumping around. Too bad I can't bottle some of their energy. I bought the belt on Amazon ($15) and just drilled a couple holes. Not sure I'd recommend the infant seat, but it would secure it if you're going to do it.
Superior 46-3230B Auto/RV Seat Belt, 74" Length, Black : Amazon.com : Automotive