> Just needs sealing or taping to make it completely watertight.
Nicely made, but ...
Can I gently suggest you test the stuff sack first? Obviously I do not know what fabric you have used to make it, but from the appearance I would suggest it is only splash-resistant.
What WILL work is lining it with a slightly larger plastic bag. With enough length to let you do the top up inside the nylon bag.
How do I know? Sigh! :-)