Question on tubing for an A-frame
 

I'm building a A-frame to fit inside an suv to pick up an electric scooter for a disabled person. This suv has no ready made lifts available (Goodness knows I've checked). The A-frame will slide out on glides about 3 feet behind the vehicle, and I'm concerned about the weight of the scooter splaying the legs of the A-frame under load (the slides offer good vertical support, but poor horizontal support). The A-frame tubes need to be 1 inch diameter at most to make everything fit. Also, the sides will follow the contour of the SUV, so they will be wider at the bottom than at the top.

Questions: how thick should the metal in the tubes be to resist splaying while picking up 150 pounds?

Is Aluminum the best material for this, or would another work better?