I made a very similar device to the above, a couple of years ago. The two plates are spring loaded together and forced apart by tapered screws. My springs were 35Kg load but there was still a movement of the plates with different telescope position. I subsequently made one similar to
http://www.optcorp.com/telescope-acc...rc6-and-8.html where the two plates are locked together and thus no variable sag, as it were. A much better design. There are various sizes at Optcorp, Telescop-express.de and Bintel (I think) and you should be able to get something suitable for your rig
Charles