zipper attachment to welded/ glued bags. Procedure??

Hi guys,

Didn't know in what category my request falls, so I do it here as it seems the most proper. Recently, I had access to an Ortlieb-like fabric. To be more correct, it is an inflatable boat fabric of the type vinyl coated polyester and I tried to make a handlebar bag.
The first attempt wasn't successful, the second is better. I used a heat gun to weld the parts. However, the following issues popped up:
1. At what stage of the making do you attach/ sew thee zipper? Is it the start when you have cut the different parts of the bag and still have not started welding them together? If so, how do you protect the zipper from melting?
2. How do you arrange the waterproofness of the zipper and the sewed fabric around the zipper?

As you can see from the attached photos (the first two photos show the first bag, the remaining three photos are the second bag), I have sewed the handlebar attachment anchors to the PVC fabric. However, I sewed them after I measured and cut the fabric and it was easy for me to weld pieces of PVC fabric on the inner side of the bag in order to make it waterproof.

As for the zipper, my first attempt was to sew initially it prior to starting the welding process. However, most of it melted. In my second attempt (the bag with the cylindrical form), I sewed the zipper last. I thought that the waterproofness would be established by making a zip-cover. But the issue was how to make that zip-cover.

I cut a longer piece of fabric to weld to the circular side walls so that extra fabric would remain. The extra fabric would be doubled so that the one side match the other where the zipper would be sewed. Doing so, I had an issue how to weld the doubled piece with also leaving at the same time a sufficient breadth for sewing the zipper. I am opening a bracket here by saying that the heat gun is with an ordinary nozzle with an air flow of 2 cm which is not narrow enough and I was not able to make the cover. I came to the conclusion that without using glue, it would not be possible to complete the project. What I did was to stitch together the doubled fabric and cover the stitches with glue.

Of course, the easiest option is to weld a separate piece of fabric above the zipper. But the reason why I wanted to make a double cover (at least during my first attempt) was to use the space for storing things like documents or other smaller items. On one of the photos there is a vertical slit on the inner part of the cover.

So I would like to know, in addition to my questions above:

3. What glue you use for Ortlieb-like fabrics? There are loads of types of vinyl plastic adhesives but the prices vary extensively. So what is the best price/quality option.
4. At what distance from the zip you sew the handlebar attachment anchors? Mines are roughly 4 cm below the zipper but the bag sags. what if you make a bag with square sides
5. The bag sags the furthest from the attachment points and it is difficult to unzip it with one hand only. So if I put something heavy inside and need it while riding, I cannot unzip it. Some people would advise me to put a plastic piece inside the bag but I would like to know how to make it firmly attached inside.
6. If you cover the zipper, how do you secure/ unsecure the cover while riding?

Thanks.