As a sprightly class 2 driver who is accustomed to regularly decoupling my buttocks from the seat, I daresay both these problems sound somewhat chub-induced despite your statements to the contrary. Seatbelt asphyxiation and being too plump to fit between the cab and trailer are not problems typically experienced by those of ferret-like proportions.
But I hope a solution is forthcoming as we know that ‘Fat Lives Matter’ too. If all else fails, I would suggest that six months of working for Palletways might solve the problem.