I went with a 2x4 on top, 2x6 on bottom, 2x6 on each end vertically and one 2x8 vertical to make up the length of the glass with a 1/2" plywood backing
Laid some 5200 down then the liner put the piece of glass on top and waited a week
then I cut out the window square in the liner and then gooped in the glass put some bags of gravel on top waited 2 weeks and...
Here's the front still needs a little attention
broke my first piece of glass not enough goo went into freak out mode and put 5 tubes of 5200 on the second attempt
Out of place pic put a 1x1 on each joint of plywood
tonight I did the corners used 3m adhesive spray 80 vinyl & rubber tried for beauty but my ventilation was lacking and it turned into get this done and get out of here. It'll be algae overgrown soon anyways.
Here's a 5 gallon bucket for reference I can't take a front pic until I get the tanks with fish transfered in