Several years ago I built a 125860 cubic foot balloon,it had 22 gores one hundred feet long,a lot of seam sealing.Through a lot of trial and error and testing,I came up with a way that is fast and if done correctly as strong as the poly.I know this because I could stretch samples to more than 6 times it's length without failure.
First you need a soldering iron,a flat price of polished aluminum or stainless steel,or plate glass about 6X18 inches and a metal ruler.
Place the plate under the 2 layers that are to be welded,then about a half inch inside of the edges lay down the ruler,then slowly guide the soldering iron along the rulers edge.Move slowly along the edge making sure you don't go over the ends of the ruler.When that's done slide the plate and ruler into position to continue the weld.Practice on scraps first and try pulling them apart to test them,you will see that it is not so hard to do.I have done several hundred feet in one day this way.
Also instead of using garbage bags that require a lot of seaming,your local building supply stores carry 0.7 mil poly sheet in rolls up to 2000 square feet,in black or clear.
Hope I have been of some help,Doug.😊