Something I read about(will try it next time) for gluing acrylic, might work for this too. Your on a much larger scale, but it could work?
Something like...
Dowel rods.
Maybe 4-6 down each long side and 1 on each end. With just a big enough dia. to prevent squished fingers.
Set the pane on them. Adjust to just right.
Start pulling out dowel rods.
For something this heavy. You could put an screw eye in the end of each dowel rod. With a string or whatever, strong enough to pull them out from under the glass with.
Just an idea I wanted to toss out, incase it'd help.