Hi Kankama,
I also have the 319K and have used it to make a sail bag.

I made this as a trial to make sure I could do it and get the dimensions correct. For this trial I just used cheap 'waterproof upholstery polyester' from Ebay. It cost $79 for 5m and has actually lasted better than expected - still looking OK after 2 years (maybe just a bit faded).
It's much higher at the mast end then ideal, but the cutout to get my sail slugs into the track is quite up above the boom, so I can't get the sail all the way down onto the boom. It's on my to-do list to make some kind of mast gate to allow the sail to furl lower, then I'll make a shorter bag. The flap around the front of the mast is closed with a zip but is a pain in the ass so I'll replace that with clips and webbing.
The 319k is a good little machine that get through an adequate amount of fabric, but struggles to feed anything too thick. This is where a walking presser foot would come in handy. If you are not too precious about stitch neatness it is possible to drag the material through by hand when it gets hung up. Be careful what thread size you use. I found that Sailrite Tex 90 (V92) Polyester was just a bit too thick for the machine. It threaded on OK and seemed to work alright, but caused massive jams. I use Schmetz 206x13 size 14 needles. There was some modification I had to do to the bobbin case to enable this. Let me see if I can dig up the reference.
Edit: Here's the article I followed about modifying the bobbin case
danhopgood.wordpress.com/2018/08/06/singer-319-needles/