The first version was too narrow, and also showed that I can't sew curves so I needed to leave bigger borders than I did. So I cut a paper template that turned out to be just the right size once I'd made the second version. I also tried pulling it closed with string but that was too hard, I needed three hands to hold the cover in place, plus one to hold the spring-loaded clamp on the string and another to pull the string tight; then with some 6mm shock cord which works but is much too strong for the task; so today I bought some 8mm sewing elastic and that's much better.
Now I wonder if a proper version of this could just sew the elastic straight to the nylon, which might make it easier to fit. I could inpick the first cover and buy more elastic to try that idea out, but it means sewing elastic that's under tension which could be fun.
|Cover Mk 1 on top of the template|
|Cover Mk 2 fillted, hooked over the mirror.|
|From a distance it looks fine|
Followup: The elastic works better than I expected. I can now fit the cover by clamping the front under the hatch cover, pulling the other end to the back and fitting it from there, running a hand down each side to the front. It's much faster than my first attempts and really only possible because the elastic is fairly week so it's easy to work with. The shock cord was way, way too strong.
The main problem now is that the channel I sewed for the elastic is too wide, so the elastic tends to pull up and out of the channel as I'm fitting it. It's definitely time to find someone who can sew properly.