Take a crash-test-dummy and strap it in ;-)
Seriusly, in the automotive branch they use bodyshaped plastic structures filled with water, which they strap to the seats ( standart safety belts ) to simulate the weight during testdrives.
Or take a big backpack, filled with sandbags and/or waterbags. It also could be easily and safely secured with the standard seatbelts.
skadi