I always keep and reuse the plastic sacks which contain 25kg of water softener salt. Normally, I leave the bags open when I use them, but today I filled about 10 bags with gravel which will probably not be used for a number of years. I'd like to seal the tops of the bags properly just like they are when I buy them, probably involving some form of melting. Any suggestions?

I always keep and reuse the plastic sacks which contain 25kg of water softener salt. Normally, I leave the bags open when I use them, but today I filled about 10 bags with gravel which will probably not be used for a number of years. I'd like to seal the tops of the bags properly just like they are when I buy them, probably involving some form of melting. Any suggestions?

EDIT: I don't want to have to buy anything to achieve this so am looking for a hacky solution. You can assume I have a very good selection of workshop tools and hot-glue gun etc. For example, my first thought is that I can probably do it by clamping lengths of metal either side of the bag and using a blowtorch to heat one or both sides to melt the plastic together.