Okay, I'm gonna go out on a limb here and guess that you're talking about the air inlet tube, rather than the upper intake manifold. If you are, go to a plumbing supply house and get some pipe insullation with the same inside diameter as your "custom intake" outside diameter. It's basically a foam tube that is slit up one side so it can be placed around a pipe.
Works excellent.
If that's not what you were talking about, nevermind.
Take care,
~Chris