The stock Keihin 32mm carburetor comes with a coolant path so that radiator coolant can flow through it in order to warm it up in cold conditions. The coolant flows from the smaller tube in the stock cylinder head, then through the carburetor's coolant path, then out to the smaller tube at the top of the radiator.

First, by eliminating the stock cylinder head and using and aftermarket one (K-fab Head), the smaller tube on the cylinder head is eliminated, so that takes care of one side of the equation.

Next, by eliminating the stock carburetor and replacing it with a Keihin PWK 39mm carburetor, the coolant path is eliminated altogether, so that takes care of the middle of the equation.

Finally, I'm left with the smaller tube at the top of the radiator. This smaller tube needs to be plugged so that I can complete the total elimination of this secondary coolant path.

My questions are as follows: What do you use to plug this smaller tube? Is there a rubber cap that can be clamped here? If so, where does one find this animal?

Thank you in advance for your time and efforts!