Thanks, a few comments to add now, at least because this sort of thing - fabricating from rubber - comes up now and again, and best I have found is buying hockey pucks. I don’t want to deviate from the original design at all.
As might be evident, I already have bought one from the dealer. You can see the photo showing a brand new item. It is looking like it might turn out that I will do this for unforeseen circumstances.
The crafts store or Grainger or Amazon etc idea is what sounds best and sounds like it is a slam-dunk - but in practice, browsing rubber on a computer just isn’t the same as holding the piece in hand. Once I manage to get in any B&M store, best I get is to randomly find some product marketed for another purpose - this is more tiresome to navigate than it sounds, and is for long-term projects because it takes so long. I am really looking for a B&M store where you can browse a selection of item and compare in-hand. If any specs of the rubber I should identify, let me know.
So any specific products that can be cited that make good rubber blocks would help here - for instance “my little pony fun craft set found at Christmas Tree Shops has a long rubber block”, or “Honda crv’s have this kind of piece to fill a void in the door sill, part number 525262” or some such is the form of response I was going for.