I wound up using packing material made up of the same stuff as the noodles.
I cut it up very carefully with a large, sharp kitchen knife and had to cut quite a lot of pieces before I got enough of them to be exactly the same height. Getting things level was the most tedious part. I placed them on my wall mounted shelf and rested the plynth on them , not between the plynth and base so they wound being 3.5" high. If you don't have the SAMA this is also good way to isolate the plynth from the motor. If you want them between the plynth and base, I think around 1.25 inch tall should be high enough. Give it a try. It's cheap, and only takes a few hours.
Another thing you may want to try instead of the sorbothane is Navcom.
I beleive this was VPI's original replacement for the springs, and when this was no longer available they touted the sorbo. Hunters use it to dampen vibrations on their riffles and arrows, They're sold as Sims Limbsavers and are available at:
http://www.cabelas.com