Butyl.
That stuff sticks to everything, and cleans off easily when you get bored with it.
I have little knick knack stuff stuck to the "shelf" ring on the
Burro.. Stuck down with tiny bits of Butyl. None have fallen while in tow.
I have the same suction cup problem and this weekend, after sticking back up stuff, I thought to put a little butyl in the cup, but forgot.