We already have test units for hand baggage, why not a test unit for the pax?
A row of three economy seats, bolted into the floor at the end of the check-out counters. A skinny mannikin in the middle, seat the "suss" pax on either side:
If the mannikin implodes or the seat belt doesn't fit, our respected pax pays more.
I'm afraid the plump really should pay more. Ditto people who foist their howling, crapping babies on the rest of us in the cabin should also pay more.
And anyone who turns up late to the airplane shoud be hit with a late charge, payable in cash, which then subsidises the bar-cart for those of us who managed not to affect the comfort of anyone else.