My two cents..
First, Uhaul, Ryder etc. doesn't let you do this. Something happens and the truck gets damaged and you'll be on the hook for the damages. BTW, buy the insurance... Your credit card doesn't cover collision damages on a rental truck like it does on a rental car. I found this out the hard (and expensive) way.
Second, I've never seen a good makeshift way of loading and unloading a car from a truck. Yes, it can be done, racers do it all the time, but you really need to do it the right way with some good, strong ramps. I've seen it done best by load/unloading to a loading dock. Of course, then you have to get the car down from the dock.
Rent a Uhaul trailer. If you don't have a truck, rent one of their small ones. The trailer will be the easiest and safest way.... probably the cheapest in the long run.