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?
How much do you want to spend? I once had some really cool Hales two-way speakers that didn't achieve focus until I placed them on chairs in my living room. There are available stands (look at stands for Harbeth 40.1) and if you want to go nuts, go to a maker of monuments (headstones) had have some granite carved.
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.

I own (4) JL Audio F-113 subs. I placed them on top of 4" maple slabs with 2" high brass Audio Points underneath. I also installed 4 Herbies isolation sliders underneath the spikes so that the subs can be moved fairly easy. I even painted the maple slabs high gloss black to match the Fathoms.

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

You might want to check with the sub manufacturer to determine if there is any down side. Raising the sub off the floor, for example, might result in reduced output since there will be less boundary (floor) reinforcement. Might make for a good no-cost experiment!