Thanks for the prompt, comprehensive response and useful suggestions.
Strangely water ingress hasn’t been a problem and I’ve cycled through some fairly heavy storms.
I’m just guessing but perhaps the contacts are sufficiently far apart or the current just vaporises it before it becomes an issue. There’s only been some slight corrosion on the xt60 connectors which are fairly tight anyway.
Heat hasn’t been a problem without an enclosure. Perhaps the motor doesn’t draw enough for that?
My previous arrangement worked very well with the battery sitting on the pannier rack and a tight bungee holding it in place. Quick release/change and secure.
The new position is currently slower change but removes the need for a pannier rack.
I’m now thinking of a beefed up slide-on connector (with drainage holes) jubilee clipped to the seat post and a bungee for extra security might do the job?
Obviously not retail-ready but functional.