Any exposure of a panchromatic film with these filters in place means one of two things (or both):
1. The filter is not doing its job of cutting off certain wavelengths at 100% (and few filters do). Some light that should theoretically be blocked is making it through. In long enough exposures, this may be enough to actually give the film some density, as you have experienced.
2. The film is responding mildly to wavelengths beyond its stated spectral sensitivity, which stacks up in a long exposure.
Number one seems far more likely to me...but I suppose there might be a possibility of number two happening, for some odd reason of which I am not aware.
To the OP, there is a 4x4 Lee #87C gel filter from Freestyle for about $30. The filter cuts off at 775 nM instead of 720 nM like the R72, so aside from the aforementioned "leakage" it would not work with Ilford SFX, but would be fine with IR820C, Rollei IR, or HIE (if you still have any). If you don't want to go the whole route of getting a square filter holder and what not, you could cut this into a 77mm diameter and pop it on top of a clear filter, or remove the glass and replace it with the gel filter.