I was running a flat DT1 filter cross the whole of the bottom of the airbox with a caged opening where the o/e filter sits, but that's torn and too expensive for me at the moment.
I've been readind some interesting stuff that Micha has been posting about the relative breathing properties of different filters and it appears that the o/e filter is just as good as any aftermarket item.
see here http://www.apriliaforum.com/forums/s...-for-the-Mille
There is a reference in that thread about the resonant frequency or something that suggest the greater the distance between the trumpets and the filter is better (paraphrasing here and probably mis-quoting). So I'm thinking about using the o/e filter with the sponge, sealing the top of the filter and bracing it with a metal strip to the bottom of the airbox, should be simple enough.