Having spent my career in a chemical laboratory I am used to going to a stock room and just signing out whatever is needed and we had all kinds and sizes of TUBING. Here is just one supplier of Tygon tubing:

http://www.usplastic.com/catalog/pro...uct%5Fid=12314

It looks to me that the 1/4" od tubing can be bought cheaply and it should compress nicely as a gasket around the edge of the glass rather than mess with a "tar rope". There are probably other suppliers but the one above should be a source of various tubing types.

Don Shillady
Retired Scientist/teen rodder