I'm having a bit of trouble understanding how your window/panel is set out.
My cousin modded a side window into his Lian-Li V1000, but it wasn't a simple square window. It doesn't really matter though. For the sake of simplicity, let's assume it was just a square window. For example, let's say that he cut out a regular square with a width of 8 in. in the center. His window was actually a regular square with a width of 9 in. So when he put the window behind his panel, he could stick tape between that 1 in. space between the window and the panel.
It actually held up damn well.
If that doesn't hold, could you always drill a hole between the stacked parts of the panel and window and just secure them with a screw and bolt?