I just went to officeworks and got a bunch of little sticky-labels for all my film backs that don't have a spot in which to insert a piece of the film box (usually the sealing bit of paper when I start the roll, most of my stuff gets unboxed before frozen).
On my Kiev 88 old-style backs (which I think are the same as true Hassies), there's the little window on the back to open to see the frame numbers on the backing paper, on them I just jam a bit of the sealing-paper under that and squish the door closed.

I do like the idea of flash-shoe mount cardboard, it wouldn't work for backs but it might be ok on TLRs where you only have one film...