Is a glass viewing window for wooden pond possible?

[Crumbs]

I am constructing an above ground pond out of wood with a pond liner and I am planning to install a viewing window. How can I make a frame for the glass that will not bow and secondly can I simply use Marine Goop to adhere the glass to the pond liner? I have bin pulling my hair out on this one for a while and need some suggestions.
Here are a pic of the wall that the window is going on. The front wall is 48" x 18h
Any help or links would be greatly appreciated!