I will make a small ND filter and glue it into position when I replace the glass tomorrow. I cut some picture frame glass but will order some tempered glass also. IIRC the ND filter was about 1 inch by 2 inches? Maybe it was closer to 1/2 x1 1/2? Can someone measure theirs? It probably doesn't need to be exact.
Just to keep you on alert, the purpose of the *tempered* glass is to prevent damage from the high heat generated by the HID lamp.