Threaded pipe
I built this for like $40 at the Home Depot. You can get 3 DH bikes or 4 XC on it. Or 3 DH and a hardtail.
The top support is a turnbuckle, it's made of a Huffy top tube, with a 1" steerer slid inside. There's a chain running down the length of the piece, so the chain supports the weight, and the turnbuckle tightens the chain and prevents the rack from bouncing.
It's very strong; the sheet metal is reinforced from inside the tailgate. You can do pull ups with it you want.
I can also open the tail gate with bikes on it and *it will stay open*. I removed one of the tailgate struts and made a locking leg - kinda like a patio umbrella with a push button.
It's all galvanized and rattlecanned, then I wrapped the whole deal in tubes, some of which are torn in this pic.... I use toe-straps to keep the bikes from bouncing in to each other. The rack is also high enough the the wheels hang above the bumper, so the bikes don't drag off-road.
Anyway, it's pretty permanent, maybe not the best for keeping resale value on a "nice" car.