Best way to keep REL sub from “hopping around” at high volumes

+1 the SVS footers.  You can pay more, but I’d try these first due to their reviews, cost effectiveness, and ease of return if they don’t work.  If not you can move on to something like Seismic Pods, which are much more expensive but by all accounts at an altogether higher level of performance.  But I think in your situation the SVS footers will work just fine.  Best of luck. 

https://www.townshendaudio.com/hi-fi-home-cinema-equipment-vibration-isolation/seismic-isolation-pods/

If your Rel is "hopping" around, are you sure you don't have the volume/gain turned up just a little to high?  I have two Rel subs in my system and I sometimes have to adjust the output depending of the disk I'm playing.  Some recording just have so much looooooow bass and at such volume.........   Herbie Hancock's Dis Is Da Drum is a good example.  I suspect the Rel sub wasn't designed to have the cabinet vibrating during play?  Just a thought.  

Build a box, fill it with sand (covered by a lid), paint it black, put it on top of the sub.

Best by far, Townshend Bars. These do far more than prevent hopping around, they actually improve bass, and also greatly reduce bass getting into the floor. Huge improvement.

Second best, Nobsound or other springs. Not as effective as Pods but at least as good if not better than all other options. 

Thanks all, for the options so far.   Much appreciated.     Jim.