You may want to add some iron oxide if you want thermite. As for small vials, I have been dipping the cap area in a can of tool handle liquid. This is some kind of plastic/rubber dissolved in a solvent. You dip the tool handle into the can & when the material dries, you have a rubbery tool handle.
Turn the vial upside down & dip. The plastic stuff dries & covers the cap/neck area with a continuous layer of rubbery material.