Thanks for posting. Just last night, I was thinking of building my own rehersal PA speaker stands- although the design you used/modified won't work for me, the materials used are getting me thinking in different directions.
They sell this rubber no slip product at hardware stores:
I put it under my speakers so that they won't vibrate and move. It really holds. I think they sell it for lining drawers. I've seen it in black, green and white rolls about a foot wide; possibly a Rubbermaid product.
I put it in between my speakers and 2"x8"x16" concrete caps they sell for putting on cinder block walls. The concrete makes sure the sound doesn't go down through the speaker stand.
You can use heat glue to adhere the no slip stuff on to the tops of things permanently.
That stuff is actually a non slip pad to keep area rug's from sliding on hard wood floors. I know because it's under the one in my living room. I can see how it would work for this. Good idea.