I have a K&N filter with the black plastic peice that comes with the K&N when you buy one. It sets inside the hole were your filter sets, and then the filter fits nice and tight under the lid.
Most people just want to sell the K&N filter, but to really make the filter fit snug you need the plastic insert. The filter isn't hurt in anyway nor the plastic insert

I want $25 for the K&N and Plastic insert together.buyer pays shipping, I wont be but a few bucks though it does'nt weigh anything. Fits 98-02 f-body