I've had my roadbook on a stiff rubber mount. Even though it was pretty stiff, it still vibrated so much I could hardly read it when the engine revs were in the wrong place or the terrain was too rough. The flexible mount sounds real good, but I'm not convinced it would stay still enough for reading the roadbook and instruments.
I've got a lot of HDPE on my sidecar. In the States there's nothing that beats McMaster-Carr
for easy selection. The stuff is almost indestructable!