Not to argue with the OP - just to ask you to review your statement: you say a hoist is awkward and dangerous - lifting 70kg into the boot. I'd suggest its probably the easier solution and can certainly be made safe. Awkwardness is more subjective.

Few solutions are going to "sit" in the boot - there's either going to be an awful lot of framework and counterweight, or you're going to have to bolt to the structure of the vehicle.