You can make a factory one work by mounting it on the out side of the side rails at the recomended height. You will just need to make the mounting points on the side rails to bolt the spring tube,cable pivot and cable end mount to.
Thank you specialtylc, do you have this set up? Does any one have any pictures of mounting the gate assist on the wall of 4' high sides? I am concerned about the assist getting in the way on the inside, and there is no way to mount on the outside. Thanks again!