Probably not much that would work better.

Thinking through this - the rubber insulator absorbs the road shock. The metal shims are extensions of either the frame or body depending on how they are mounted. Adding possibly a nylon or even a urethane washer might give you some additional cushioning and corrosion resistance, but anything soft like neoprene will eventually compress then you would be back to the project of body realignment (a subject I have gotten to dislike intensely in the past few weeks)

On my '31, I started with the normal rubber cushions, which are about 1/8" thick, then did the rest using 1/16 and 1/8 inch front end alignment shims - now the right side needs a wee bit of touch up - 4th time