There are two types of MPP film insert that I have come across. There is flat type where the metal surface that the film sits on is thin and totally flat and this is the one that is most likely to be suitable. The other type has a corrugated surface which makes it thicker. It also occurred to me that maybe I could nip the edges of the septum in slightly with a small set of pliers but I am loath to do this really.
I look forward to the results of your brainwave Ian.