For vibration absorption properties, it's hard to beat the latex rubber foam sold in the hobby shop. Receivers are of such low mass that velcro seems to do the trick for me. For fuel tanks, i sometimes make a doughnut out of a piece of pipe insulation and glue it to the back side of the firewall where the neck fits through it. That holds the front of the tank in place and then a piece of foam behind to hold the tank snugly against the doughnut. Other foam items i've used are foam weatherstripping, packing material, and drink coozies.