What can I use to make a suction cup stick to a non-flat surface that will not permanently damage the wall?

I know using a suction cup on a non-flat wall defeats the purpose, but this is what I have to work with. We bought a removable shower head. The holder requires it to be suction to a flat wall. We do not have flat walls in the shower. I there an adhesive that we can use that isn't a commercial grade glue that will make it stick?