oh so we are talking about panel filters here? of course there are drop in panel filters for skylines... k&n, blitz, hks, trust, apexi, redline... every brand has them. You can get the k&n or redline from Autobarn, or the other Jap stuff from performance shops... this shouldn't be an issue at all!
the challenge is trying to find a air box containing a pod filter like mugen do for hondas... they would work wonders, keeping out engine heat and saving you from a canary (provided cops don't open up ya box)