I have an old set made by Millet that are more of a stiff foam with a slick rubbery covering on them that I've used for years.
My oldest just recently found a set made of a more dense rubber like you describe at Walmart made by Birchwood Casey.
I believe he paid just under $20 for them.
I have three sets of older Hoppes rests with leather sand bags for myself and my boys, but these just pack so easily in a range bag and don't weigh as much as my better looking half when I'm carrying the bag with all three in it.
I typically never use the rear bag or rest anyway when I'm actually shooting.
It's more or less there for supporting the rifle when I'm not.
So.... Birchwood Casey... Walmart at least did have them earlier this year.