You can start with school white glue and graphite powder (from pencil),
mix it and apply a small disk directly to the rubber. It will hold for
some months. The graphite quantity into the mix will define the contact
resistance.

For final results, try another strong glues as Araldite or some
epoxies.  Copper powder, as well silver, bronze or brass powder into the
glue mix does marvelous results.  The ideal is that the "pill" should be
a little flexible to make good contact at the pcb fingers every time.
Remember that some glues as Silicon "wraps" the powder particles in the
mix, electrically isolating them

Grab a big truck and goes after "not working" nintendo or sega
controllers, you will find them in large quantities around, probably you
may have a couple at home. They are quite strong and the "pills" are
doped to stand kids abusing it. Just remove and "work" those pills
before glue it to your rubber keypad.