This is, for those who don't, a great option - but I'm already carrying either a "messenger bag" or a small rucksack.

To put it in different terms, my overall inventory is fine, but the problem is that hot weather causes you to lose all your "quick inventory slots".

Imagine any computer game where you store a selection of key items in the quick slots, which you can access with the 1-9 keys. In summer, only the shortcuts 1 and 2 work, and for everything else you have to go in your main inventory, so you keep swapping which items are in 1 and 2, but you have no consistency in what's under each number, so you're trying to get your train ticket out, but you end up with your house keys.