ORIGINAL: drube
I tried to find Cabosil locally with no luck, but the west systems stuff I can find here at west marine.
Charlie,thanks for the link. Lots of info there.
DRUBE,
Look up "FUMEDSILICA" on the internet. You should get 100's of hits which will include Cabosil and a bunch of other trade names. Fumed silica is an excellent thickener, possibly the best. Fumed silica is extremely lightweight and fluffy and it should be mixed in a container with somewhat tall sides to keep it all together
Another approach for your project is to use JB-Weld. It is ready thickened as it comes from the tube. Good luck