For holders that don't have protrusions that will bear on the glass directly, and if the spring arrangement of the ground-glass holder has sufficient travel to accommodate the thickness of the back, then there's nothing wrong with just sliding it under the ground glass. I do not know whether the back on the Sinar will accommodate a Polaroid 405, aside from the issue described by the OP. It will absolutely accommodate a Fuji PA-45 in that way, though not with an abundance of margin. I just tested it. The PA-45 back is about 1.07" thick.
Rick "noting that the bail tab has a 1" travel, so lifting the back itself is required with the PA-45" Denney
Bookmarks