Honestly, I don't think this is going to work out for you.
The entire weight of the battery is on the four plastic screw mounts on the cover,..
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The straps are pulling the cover closed, but there's still going to be vibration and movement at the screw mounts.
You could just try it and see how it goes, and if it doesn't work out you can mount the battery to the down tube.
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Mounting to your frame is a much better option and it doesn't look that bad without the cover.
You might be able to cut and trim the cover and fit it around the battery for a cleaner look after the battery bracket is installed?