Any soft solder will work on the flask. Its not going to bearing much weight, nor is the seam subject to much stress. The real problem will be preparing the flask seams for the solder. Do clean out the flask and dry it completely to remove any and ALL moisture from it. You will want to use some kind of acid flux, but getting into the seam is the hard part. Use very thinned needles to push the flux into the cracks.
It would work better, if the flask were opened up so that the edges over the entire seam could first be cleaned with fine emery clothe, and then flux added. Then each side could be flash soldered. When the two halves are put together, then heat could be applied to melt the soldered surfaces so they adhere to each other. That technique will give you the best seal.