I believe the OP is looking for 'Vibration Isolation' for a fairly heavy head, not a grip-pad to keep a class D head from sliding.
I can sympathize, and it's not easy to find Iso-Pads that are for a 30lb or 50lb piece of gear. Vibration Isolation Pads or feet are easy to find for something that has the weight of a home-stereo turntable etc., or for something the weight of a shimmying clothes-washer or shop air compressor.
Upthread, @EvilJ seems to have offered a brand of vibration absorption/isolation products that are tailored to a variety of applications, some suited to electronics like audio amplifiers, even targeted to intended "working load".
I've had and have some tube amps and even racked solid-state setups that have needed isolation. Folded black washcloths or even doubled-over wetsuit-type material has worked for me, but something purpose-designed that could even stay attached for transport certainly would be nice.