You could try fashioning an acrylic bracked that hangs on the back of the aquarium that the powerhead is connected to.

Or you could silicone the powerhead to the aquarium, but that is more of a permanent solution. There are also the kind of powerhead that is used in saltwater tanks that are held on by magnets, but they are pretty spendy.

You could DIY a magnet system. Hard drives from computers have super strong magnets that would hold a powerhead through up to a 1/2" of glass. Superglue a magnet to the power head and CAREFULLY mate it up with another magnet on the other side of the glass. I have a couple magnets that have to be slid apart to separate. It takes quite a feat of strength to pull them apart.

 

They also sell magnet suction cups. I wonder if a magnet would be problematic for the impeller. IDK for sure, but seems like a magnet could do something crazy to the metal within the unit.

 

Many powerheads utilize magnetic holders - like the Hydor Koralia line.

You can also buy them specifically for other brands.

 

I place two wooden dowels on both side of the power head like a "feet" and tie them all together with velcro strap. Push altogether into the substrate as deep as you can. I have never been worried of suction cups from the power head.

I think dowel size is 1/8" dia, maybe smaller. You can get this at HD or Michaels hobby shop.They come in 24" long so just cut to your size you want.
Replace when dowels comes to rot. They are cheap.

 

You could try pulling the suction cups off the powerhead and putting them in a cup of water and then boiling them for a few minutes in the microwave oven.

 

Thanx 4 the great ideas!