How can I make better use of the dead space under my bottle cages? 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)
Good question - I've wondered the same.

I looked at the Specialized SWAT toolbox for inspiration, but it only fit certain models of frame.
https://www.specialized.com/us/en/equipment/bike-accessories/swat-technology/c/swatstorage





Another option is to mount things directly to the waterbottle cage mounts.  Here's another specialized product to do exactly that:


https://www.specialized.com/us/en/emt-cage-mount-mtb-tool-for-right-zee-cage/p/132369

Downsides of this is that it only fits their multitool and nothing else.  Also exposes the tool to a lot of dirt and muck that it wouldn't see if higher up.



I have tried a solution of using a frame mount bag right at the front of the main triangle.   Advantages are that its forward weight so helps me keep the front wheel down on a steel climb, and I can put a USB battery in here for powering my gopro on long rides.




Alright this bag was measured for another bike so its a bit saggy here.  The pump hook gets in the way, and the downtube is much smaller and a different angle from the designed bike.  Flaws - the zip is the wrong way around, and should close forward so cables can come out easier.  Velcro is all wrong for this bike.

I also use a top-tube bag for more storage when on a long ride.   That holds snacks and my phone running on another USB battery (because its annoying when strava stops recording.)