I 'm thinking that you could make an upside down U and stub the bottom ends into the frame rails with small receiver like hitches. Use like 1" OD tubing for the "U" and bolt or
weld 1" ID tubes to the frame rails. Pins would hold in place during use and you would miss the exhaust system. Hang your bikes or whatever on hooks welded to the tubing. Keep it lower than the rear window, paint it body color, and take it off when not in use. You probably wouldn't see the receiver ends under the car. A decent tubing bender could do this for you cheap.

Joe