In order to make the seams in the bladder watertight I decided to weld them using the technique described by instructable user Aleksi (https://www.instructables.com/id/Welding-plastic-ba...).

The two pieces of plastic to be welded together are sandwiched between two pieces of oven paper and then the soldering iron is run over the top piece of oven paper in order to melt the plastic together. CAUTION: Remember to always be aware that burning plastic could potentially release toxic fumes and therefore this should be done in a well ventilated area and with proper precaution.

Be careful not to touch the plastic directly with the soldering iron as it will create a hole!

Be sure to leave an area of seam open in order to be able to fill it with gel later! I left the neckline open.

I chose to create my welds with the sandwich of oven paper and shower liner held against a copper tube in the hope that this would help the lower sheet of plastic heat up and create a more even weld. You will probably also want to practice the technique on some scrap sheets of plastic. I found that lining up my marking on the top of the copper tubing then using the side of the soldering iron to go over it with very small circular motions worked best.

I also created horizontal baffles in the bladder so that when I added the ice/gel it would not all sink down and gather at the bottom after melting.