Traditionally, I have used frosted scotch tape as a ground glass. Laying a few pieces across the opening for a conventional camera is no problem. The curved surface on a Panoram is a little trickier. Maybe use a very thin sheet of clear plastic and cover it with several strips of tape. Then bend it into place. The clear plastic sheet has to be thin enough so it does not affect the focal plane but stiff enough to remain taut when curved around the back. You can probably salvage a scrap from something like an envelope window (a big envelope window)