Warm the rubber in hot water use soapy liquid spray and thick string or leccy flex, even get para cord ...
Get somebody else on the outside holding the glass for you in position, not too much pressure, just enough and if you use quarter inch thick rope or whatever:
start from the top of the rubber down to the bottom, make a loop and continue putting the rope on till you get back at the top where you started from.
pull the rope, both ends towards you till the rubber lips over the aperture for about 6 inches.
Now, see that loop you made at the bottom?
Start to pull that, slowly.......make sure the other person is applying not too much pressure!!
Make sure that you try to pull the rope evenly at top and bottom, then just pull the rope, taking your time around the corners.