The restraints are the whole idea behind the X-Car. It's designed from the beginning as an OTSR-free looping restraint. The way it works is slightly kind of like a European handcuff -- it's an elliptical curve that passes increasing through a ratcheting catch, thus tightening, and yet fitting more or less symmetrically about the waist.
It could stand a bit more padding, but it's a neat system. And I've been inverted in one for 5 minutes before.
Someone has a system somewhat like this, where it's a standard lapbar, with a translating ratchet that tightens the entire structure. It's less elegant, but seems to work about the same. I don't remember who had that, though. I want to say Compounce's Thunder and Lightning does.