Need a malleable solid which dissolves in HCl, acetone, or water
If the cavity in which you insert silicone is not air tight I would use and air pump to blow in the tube. 

The easiest chemical I could imagine using is a low melting point salt such as ammonium acétate. If the tube is removable you dip in the liquid and upon solidification you can remove the excess. It's water soluble. 

Sugar toffee will have a similar behavior and is malleable at a certain proportion of water. 

Plumber use bread.