My son found double thumping difficult on his Fodera Monarch, because there was too much distance between the string and the body (the top) of the bass.
So I got a sheet of clear pick guard plastic (the stuff that is sticky on one side) and cut a shape to fit just around where the thumb contacts on the bass....I then made a little "pick guard" by sticking 9 layers together for the correct thickness. This completely worked as far as ease of double thumping goes.
This is the result---check out the double thumbing right at the very end of the recording!!! -
http://www.youtube.com/watch?v=SsfrnbfbAoE
You can see the "pick-guard" in this vid if you look closely-
http://www.youtube.com/watch?v=OnkZmOqTC-s