Discovering https://www.youtube.com/user/EnchantingMusic David Hicken motivated the lion kingdom to look into this again.  He's probably on his way to being widely covered by better musicians, but for now, there are a lot of poorly played good ideas which no-one is covering.

The #1 problem is isolating the keyboard from the floor.  With the human population moving to condensed vertical housing, this problem will be solved by commercial solutions in the near future, but for now, there's just dreaming.  40lbs have to be supported by 4 corners.

Bubble wrap previously was the best solution.  6" thick columns with 1" diameter bubbles were required.  It quickly flattened, took a lot of space, & had to be replaced.  It allowed a lot of sideways motion.

Pool noodles look good, but probably are too stiff.  Any foam is going to either be too stiff or compress over time.

Compression springs of the required diameter to resist horizontal motion are expensive & many would have to be tried.  Too stiff & they don't cushion at all.  Too loose & they flatten.  Too narrow & they flip over.

The latest idea is pipes with repelling magnets.  Magnets are cheaper than springs.  They have no minimum pressure before they start cushioning & they offer cushioning all the way up to bone crushing pressure.  The pressure can be adjusted by stacking more magnets.  The tubes would need plungers with some kind of felt to defeat friction & match the diameters.  The magnets need to be attached to prevent them from flipping over.

The #2 problem is lowering the stand, manely a lot of drilling & sawing.  The #3 problem is sideways motion of the stand, but perhaps some struts would do the job.