Yesterday I installed the K&N filter HD-0800, on my 2007 SERK. The SERK has a round cover. What a poor thought out design. The cover has a large spacer welded to the incide to avoid the rear of the cover hitting the two breather housings. This is why several magazine reviews about the SERK have noted that the air cleaner interferes with your leg.
As already noted, HD put a thick rubber gasket about 1/2" on the top of the filter to make up the gap created by the added metal spacer. If you restick this to the top of the K&N, it works fine. You really do not need a gasket per se on the top (outboard) of the filter because no air can get into the filter on the top. All you need is a spacer in two or three places to help support the cover.
Did anyone foind away around this extended mess? :-/