There's an annoying/useful bit of space under each of the bottle holders on all of my bikes; I currently strap a spare tube there (which keeps the right tube on the right bike). The tube is in a bag but the bag tends to snag and rip, and dirt/grit gets in. It also doesn't look very nice. I should be able to make much better use of this space, to hold more than just a tube, and hold it better
I don't believe there's a product on the market, so I think I'd like to repurpose/make something. It should be secure against falling off, and keep the dirt and the worst of the water out.
I've found this winter that on long rides (200km+, essentially unsupported but with occasional food stops) the bike is fully loaded. I don't really want to fit the rack/panniers (weight and drag) when I'm already pushing myself quite hard, and can't add a handlebar bag because of my lights.
I've already got a top-tube bag (snacks/gels, backup battery for phone/lights), saddlebag (first aid kit, foil blanket, more snacks/gels, spare tube, jacket and shoe covers strapped underneath if not on me), 2 water bottles, tool case in 3rd bottle cage, jersey pockets full (2nd breakfast/litter, money/cards, any extra warm layers)