What is a good base for speakers

Actually they will sound better with springs. Townshend Podiums work best but you can DIY two ways. Easiest is Nobsound springs, they are adjustable for weight and cheap, only about $35. But they work. Slightly better but a lot more work is to build a platform similar to Townshend Podiums and use your own springs. I've done both, DIY platform under my Moabs, Nobsound under all the rest, before upgrading to Townshend.

Should probably go without saying but if Moabs rocking on springs isn't a problem (it's not) then it won't be for yours either. They don't actually rock, not when playing music anyway. That's only when you make them rock to show off that they are on springs.

If they are the open backed metal stands add stick-on footers in a 3-point configuration (2 in the back and 1 in the middle of the front cross brace).

They sell little rubber stick-on footers @ hardware stores and online.

Whatever the stands are like 3-point/tripod support should help stabilize them.

DeKay

I use some pretty decent amp stands for my B&W 804D3 speaks. Got them from AGon member Solidwood (Timbernation). With screw in cones came to $310. Had to drill them to secure but didn't take long.
Also, you could visit a Bed-Bath-and Beyond store and look at some big cutting boards.

Drill

any experience with wood butcher block, granite or slate cutting boards as a base? More DIY than expensive.

I have experience with all of these, which is why I recommend springs. Nothing else even comes close. And I am pretty darn good at DIY
https://systems.audiogon.com/systems/8367 Yes I built the table and stand. Springs are totally the way to go.