The trick to ramp sliding on you is to cut a piece of plywood 6' longer than the ramps and put a 2x4 cleat in the front .
When using the ramps, you place them on the wood with cleat foward ..now thw tire and weight of the car is on the plywood before going up the ramp incline .. the weight of the car stops the ramp from moving..
..and the two plywoods store easily anywhere ..
I am surprised ramp companies don't make the ramp with a 6" flat entrance, as that would do the same thing... you will notice that the ramps only slide until the weight gets on the incline..........
I have never seen any, but maybe someone makes one...
Anyway , a simple cure for the fault............