One of my hot-water bottle covers needed to be replaced - it was a simple affair, but didn't cover the spout (that could be very hot) and had a cord to tighten around the neck, which wasn't easy to undo.
I thought through what I wanted.
A cover that was snuggly,
easy to remove for washing,
easy to use by leaving the bottle in when filling,
and not fiddly to make.
I came up with this:
It did take a bit of fiddling around, especially using the bias binding on the edges and through all the thicknesses. I probably didn't have a big enough eyed needle and it would occasionally skip stitches.
So, do you think that this is an idea to market?