Yesterday I installed a K&N model RU-3590 Universal Rubber Filter in the mouth of my airbox. The filter came with an angled rubber flange (where the red RTV is), which I sliced off with a utility knife. The O.D. of the big end of the filter is 3.8", so it slid right into the mouth of the 4" O.D. airbox. The filter is roughly 8" long and tapers down to roughly 2" O.D. I floxed up a circular "stop" behind the big end of the filter to keep it from sliding further toward the carburetor. A friend machined me an aluminum keeper ring which is screwed into the airbox at the front. A short section of 4" SCAT duct will connect the airbox to the ram air intake in the lower-front of the cowling. The 2" SCAT duct is the carb heat which enters at the rear of the airbox. It looks like it should work: