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?

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!
I have a pair of JL Audio F113V2 subs and in my former home (in a room with terrible bass) I built some stands using 4 x 4's and sturdy metal hardware, and that reduced the boominess in that room.

I'm about to move into a new home and if I don't need them, will probably try to see if someone else can use them.
What is the reason for doing this?  Does it have to be 12" or are you just trying to get them off the floor to keep from passing vibration to another room or something below your room?  If you just need to get them off the floor, consider the Auralex Subdude - https://auralex.com/subdude/
Turnbowm has it right. You do not want to take your sub off the floor. You will just make them less efficient forcing you to raise the volume distorting the amplifier and the driver. Subs also should be up against a wall or in corners again for efficiency reasons and acoustic reasons. You decrease room interaction this way. Subwoofer manufacturers put their subs on the floor for a reason!
Was in Home Depot today and saw some inexpensive but sturdy stands for water heaters. 21 X 21 X 13 inches. Welded steel, $40 ea..