Raise my subwoofers off the ground approximately 12”?

I’m looking for some sort of subwoofer stand to raise my subwoofers off the ground about 12”.

Any ideas for building something or buying?
My REL 528 has a passive radiator on the bottom. Do I want them raised up? Any advice?

May sound silly, but maybe try a pair of these at $5 each first to make sure you like the performance when raised.  Then go all in.

Get a piece of Baltic Birch plywood (the stuff is pretty stiff), and if you want to go nuts glue a piece of MDF onto it. Even better, put a layer of ASC WALL damp between them (the resulting shelf will be very non-resonant). Add a set of bolt-on legs of whatever height you desire. Try different heights to find the optimum woofer location.

If you want to isolate the platform from the floor (and therefore the sub along with it), put a spring under each leg, or between the sub and the platform. If cost is no object, consider the Townshend Audio Seismic Pod, a superior isolation product employing springs in a patented design.