Brian, 1/4" sounds a bit on the thick side. If it is normal float glass, it will reduce the U.V. light quite a bit. Just as an experiment, try using the thinnest glass you can find that is available (single strength window pane), even if it does not give the compression of thicker glass. This may affect print quality adversely, but it will tell you if there is a problem immediately from glass thickness.

What size prints are you making? A sheet from a regular picture frame that is the correct size will work. You could also try leaving one edge of the print and negative exposed to light without the glass, to see about times and exposures as a comparison test.