there is a blue bottle of stuff called K-Seal. that stuff seems to work pretty good, and might work for what you are doing.
the "roadkill" guys used a few bottles of this stuff to seal up a smoking head gasket on one of their episodes, and it stopped their severe leak [along with a new radiator cap]
of course they beat on the car severely, which i don't think you are going [or planning] to do.
you might want to try some of that stuff. sorry i don't have a link.