I have transported hundreds of special needs students in wheel chairs. There are a number of different companies that manufacture wheel chair securing systems for use in school buses. Many of them include lap belt/shoulder straps plus wheel chair anchors and straps.
I would start my search there.
2005 Monaco Knight 40PLQ; Cummins 8.3L ISC330, Pacbrake, Allison 3000, Roadmaster RR8R, ScanGauge D, 2004 Kawasaki Vulcan VN750(Geezer Glide) on a Versahaul carrier pulling a 2013 Kia Soul+; 2.0L, 6 speed Sport shifter(great car) on an American Car Dolly(great dolly.)