I wonder if you couldnt just take an extra corny lid, put the gasket between the lid and the top, and hold it there until you got a good seal from the vacuum......
I have uploaded some pics to my gallery. I'm having some trouble getting them in the post. Hope this works.
This is the pump working on boiled water. I boiled the water then cooled it to remove any gasses for the following pics.
Shows the beginning of boil. This pump doesn't produce enough vacuum for a full boil but the AC pump I have used in the past will boil the flask dry in 1/2 hour. A bit too much for this application.
This shows condensate in the tubing after turning the pump off. The tubing was dry before the turning the pump off. As soon as the tubing lost vacuum the vapor condensed to form the droplets.
I split the stopper where the thermometer was thrust through so these pics don't show the thermometer.