I wouldn't just buy a magnet and use a magnet.
I glue a magfloat to the powerheads after trying to get suction cups. The suction cups I finally found I had to cut the top nub to fit the powerhead and had nothing but problems. The plastic and velcro offer some protection from extra metal leeching into the tank. While it may not be the perfect solution, I've been very satisfied with the results.
With the magfloat superglued to the powerhead I am easily able to move them around when scraping correline algae off the back glass.
Denise M.