In article ,
bob prohaska wrote:

....

Suggestions for a seat bag would also be welcome. For the moment
I'll put the tools and spares in the pannier baskets, but that's
dangerous as they can be removed and forgotten. Much better to have
necessities permanently living on the bike.

Not quite what you're asking for, but a tool case that fits in a
bottle cage might be suitable. Something like:

https://www.planetx.co.uk/i/q/BO365S...cage-tool-case

Can be easily switched between bikes. And is *visible* so you can
easily check it's present before setting off for a ride.

Of course giving up a bottle cage might not be acceptable,
especially if you're in a hot climate.
--
Dennis Davis