Stock Cessna seat-belts can do a reasonable job of restraining skydivers, provided that you route seat-belts between harnesses and bellys.
The best restraining method invovles wrapping special, skydiver-specific seat-belts around the hip joints of their harnesses. American operators of jump-planes changed their attitudes towards seat-belts after a couple of fatal crashes in 1992. Most now have "Hooker" belts installed (aka. the "tag-line" belts that are standard on PAC 750XL and Quest Kodiak). Jack Hooker (Hooker Harnesses of Illinois) invented the single-point restraints now used in most North American jump-planes.
Pssst! I have also invented a couple of other methods of restraining tandem students ... but please don't tell Transport Canada.