You will use titainium wont you Bill?
....seriously though.....did I just say that, I have been doing some research on what filter to use in the RM250/370/400 ABC, PE250 BCN models for the alloy airbox that we are going to make (well I'm not actually making then....I guess I'm a ideas man). As anyone who has one of these models will know apart from being the shitiest airfilter to change in the world (ol' tojo is still laughing at that one....I treach weeze westerner ruckleheads a wresson), most of the boxes are either damaged, farked or missing. When you look at a std air filter the sides touch the airbox (not a good thing for airflow), the airfilter in a RM250/400 N/T is nearly half the size (went like a bat out of hell mind you), so to replace it with something modern/cheap/available a current YZ85 filter and cage will do the job perfectly, its roughly the size of the N/T filter cage (slightly higher), nice cone shape so would have clearance in the airbox(the frame shape and exhaust dictate the size and it cant be made much bigger unfortunately) and promote good flow. Now after all this babble, I reckon a YZ85 cage will fit Bill, you will just have to trim the inner retaining bolt sleeve, the cage is about 10mm higher but I dont think it will matter and YZ85 filters are plentifull, go down to the Aliens and see if you can flog one. cheers.