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Plain, thin plastic sheet is the best for a dob. And longer legs.
I've come to see that this is the best solution from experience.
Consider the action of your dob on a hard surface - the action is steady as the mount is working off a solid surface.
Now, put the mount on some spongy surface, lush turf, foam rubber, whatever. First note: the little stumpy feet usually found under the ground board are not long enough to reach through the lush turf to the hard earth. The result is the turf is actually pushing up against the underside of the ground board, just like placing the mount straight onto foam. The result is that the turf/foam introduces its own set of forces and backlash to those normally needed for the dob set on concrete. This effectively more than doubles the force required to get the scope to move, and you introduce a horrible amount of backlash when you release the scope, and the turf/foam pushes back to restore its shape.
Foam rubber of any type is the worst solution. Put the foam onto turf, and the scope on top of that, and you won't be able to get the scope to work at all. No crap.
Thin plastic sheet, garbage bag cut open, builders plastic, plastic cover wrap of a mattress, what ever, will give the least amount of undue influence on the action on the dob mount.
But this too won't overcome the problem of setting down the scope onto turf. The plastic sheet here will also be detrimental to the scope's action. There are only two solutions to this - 1, move the scope to hard dirt or concrete; 2, give the scope longer legs.
Replacing the crappie rubber door stops that are typically found under the ground board for at least 4" long legs is the best thing you can do for your scope. Not only will these legs be able to penetrate more easily through turf, but will also reduce the effects of dew on the underside of the scope.
Have a look at the before and after pics of my 8" dobbie. The new 'leggie' mount has no problems with lush turf, and being made properly, damp turf is also never a problem. For this f/4 dob, the longer legs also raised the position of the eyepiece to a better height, so all I need to use is a humble chair, and no more kneeling on wet turf.
Mental.
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