[Mike V. Florida]

I don't think you would be able to make one and fit it into the neck of the radiator and then position so the overflow tube goes through the top.

Each car seems to like a certain water level. If it is a real concern to you try this;

Fill it so it just covers the tubes.
Drive it for a set number of miles, say 25. That should be more than enough to get it up to temp and then some.
Measure the amount of water it takes to fill it to just over the tubes again.
Drive it again for the same number of miles.
Measure the amount of water it takes to fill it again.

If it only takes a pint or two, it's ok in my book. If it takes more then try this;

Bend the overflow tube towards the back of the radiator (towards the firewall). You may also go to a hobby shop for some model fuel tubing and put it over the overflow tubing and up under the cap.