In aliexpress you can find foam pieces to be put under the spacebar. These 2 products:



and



Has anyone used one of these products? The design file to cut these foam pieces are opensource (can be found in github or somewhere else)?

Design file?!  Kids today...

Take a piece of foam, put your spacebar on it and put something heavy (but not too heavy - will depend on your foam density) on top.  Leave it for a while until there is a semi-permanent dent then remove the spacebar and using a sharp knife and metal ruler cut just inside the dent.  Does it fit?  Great, see if you can hear a difference.  If not cut a little more off and try again.  Repeat as required.

Don't forget you're not going to be able to see the finished piece so it doesn't have to look perfect, and that includes having square holes.