It is just rubber. Don't select anything too thick otherwise it will be very hard to get the pumps in and out of the tank.
The rubber I had was maybe 3/32 inch thick. I had two pieces of rubber but found that if I had two layers between the two pumps, the total width was two much to fit into the tank. This is with almost 1/4 inch of material already removed on each side of the pump access port.
Also, I have a pair of walbros with hanger and -6 fuel hose that I don't need and am going to sell.