my cfm box came used with a spectre filter, i replaced it immedietly with a uni, i'm not a fan of the k&n style filters, i then try'd to use it in my lil buddies 300ex's stock box, and banged it up a little trying to force it in there, but should still work, it also had some silicon cut and placed in it to reduce it to the 2 1/2" inlet of my box, so you'll either have to get a smaller diameter opening, if they make one, or really reduce it down to fit the carb, which is only 2 1/4" o.d.
the best way to do this is use a velocity tube, it straightens the air flow for better results and easier tuning, that will also allow you to reduse it with a fernco fitting or one from here...........
siliconintakes.com look in the reducers page
this spectre i have could be yours for free if ya wanna pay the shipping, i have no use for it, it looks as though it's never even been oiled, which i would suggest doing,
and slightly bent up, but should get ya through the winter till you get a box, i saw them down autozone for $29, so it could save ya a few bucks
it's also possibe to just get the proper size uni from ebay that'll fit the back of your carb....2 1/4 inch