Guess I came into the middle of an ognoing project without realising the goals. Apologies. But, that said , it leaves me with a few other questions.
If, as you say the box leaks, and the flow is from inside out, then that must mean that the air reaching the intake maniforld is not fully cleaned. It would also mean that the stock set up would look "artificially " free flowing, since some of the incoming air can bypass the resistance of the fillter. Then that may mean that even if you succeed in the quest for getteing a freer flow filte to fit , but simultaneously cure the leak, the only outcome may be cleaner air getting to the engine but no faster (since the total riesistance of the "performance " filter may equal the combined resistances of the stock filter plus the air leak. Just a thought. Nigel. Going direct to intake outside in would give a direct measure of the filter itself.