In my case, the stone required to hold down the foam island was as big as the island itself.
Even if I could have attached the island to the suction cups more securely, I don't think I'd trust suction cups to not let go or slide up over time, at least those small enough to be inconspicuous. Magnets with a non-slip coating should work better. Lead or bismuth weights embedded in the island can cancel buoyancy with a size I've estimated at about 10X smaller than the foam, so easily concealed within.
Life happens. On semi-hiatus, checking in infrequently.