Hi Richy,
I managed to fit one behind the gear box on my wife's '51 Flash but it has a non standard rear mudguard. I do not think it would work with a standard one. However, Paul Goff does a kit for fitting in the toolbox on the swinging arm models. I do not know if the plunger tool box is the same depth as the swinging arm but the filter is the standard 78mm dia.
Dave.