In a case like this, I'd get a package of the Bars Leaks tablets at any parts store. Dissolve them in about a quart of hot water, and then add the whole lot to the radiator fluid with the engine running, and right away take it for a 10 or 20 minute drive.
It's not an elegant fix, for sure, but it will likely get you through until Spring, when you can do it the right way and flush the sealer out again.
The important part is to thoroughly dissolve those tablets in hot water first, don't just drop them in the radiator like they tell you to. BTDT.