I think the multi-fold shower door on my Four Winds Motorhome is probably pretty typical. It works fine and I like it as it is much better than the plastic curtain I had on my old MH. But the door sli...
So thread a steel bolt in your aluminum frame opposite where you want to place the magnetics, or epoxy a small steel plate. Optimally position them 1/4 way down from the top and bottom of the door.