Without the glass will the rubber fit the hole , if so place rubber on the glass and put lots of dish soap on the rubber inside the channel same place as your 1/4 " string your need to A have the top off or a nice pal to press it from the outside and lip the bottom of the rubber seal on the lower part of the opening, now with a firm pressure but not so it won't move or allow the rubber to peel up/in from your pal .The string should be at the bottom both ends now slowly pull the string and the rubber will peel open as you go around from both sides ! do the top last holding the string so its tight pull inward at all times.
.or use an innertube just do same lip rubber on the lower lip on the opening and inflate to hold the glass and rubber inplace .