This is a circular, flat bottomed 7L drybag with a PP rib insert. Using a PP rib does three things, easier roll down, better seal on the roll down and doesn't blow open when pressure is asserted on the sealed bag (if you still have a bunch of air inside). If you dont think your machine will go through the PP sheet (its quite soft so dont underestimate your machine, use a pointed needle not a blunt one).
Things to note: The taping of a circular bottom with PSA tape is not the most elegant, but it does the job (100% waterproof). If you use a PU coated fabric, use a sealing tape that uses heat and apply it to the PU coated side of the fabric (tape and iron for this is show at 4:26 mark), which should be on the inside of the bag.
Any questions please leave them in the comments on youtube and ill get back to you.
As always, free A4 and A3 printables here. I have included the illustrator file so you can amend the drawings if you would like.