Mike, the RX-4040-1 is another K&N X-Treme filter and it has a 4" outside diameter base.
That's good info John, I neglected to look at the outside base diameter of the RX-3800 which is 4.5", but, it has a 20 degree flange angle to it.
Maybe I could stack it offset, mount the filter direct the TB furthest to the rear and bring them out further toward the front.
The flange length is 3" so I could conceivably make that work but there isn't a whole lot of wiggle room on the mounting surfaces, unless I cut into the flange to get it lower on the TBs.
I wonder how the RC-3680 would fit since it's a tapered oval, big ****er though, 6 inches long and no angled flange so I'm worried about my knee hitting it all the time.
Maybe the SN-2520 or SN-2510...
Decisions, decisions.