Shortest water bottle?

Can you not mount the cage lower? Maybe with an adaptor. A 500ml bottle should fit otherwise.

https://www.amazon.co.uk/Topeak-Unisexs-Alt-Position-Cage-Black/dp/B084W8MWKP

Hoy 400ml is tiny

Topeak bar/seat post cage holder?

Rather than looking for a super short water bottle you could try a water bottle cage adjuster, which would allow you to position the cage lower down in the frame. This would allow you more scope in terms of bottle length.

Something like this:
https://www.ebay.co.uk/itm/154416232697

As already said move the cage lower

My Planet X Tempest (size L) bottle cage mounts are stupidly high, so much that my 750L bottle is awkward to fit in the cage

A side load cage can help as well as you don't need any space to pull the bottle upwards.

Personally I'd be looking for a slightly smaller frame bag as that thing is enormous!

Fidlock bottles

Shortest “normal” sized bottle (or around 550ml) is an Elite Fly

Saw on Enduroist who’d melted his bottle to fit into the gap on his Orange Stage 6?? frame.

🤘

I use an Elite 350ml which is small all round as I only do short rides but guessing not big enough for you in capacity?

There's loads of space there to mount a bottle on the dow tube, use a Wolf Tooth B-Rad or similar to move the cage down.

Not directly related to the OP's problem but it did get me thinking..  given the relative size of frame tubing generally these days and downtubes in particular, wouldn't there be some sense in someone designing a range of bottles and cages that are shorter and fatter than the current standard.?  (yes, yes I know it would be throwing away one of the last true standards in cycling!!).

With bikepacking bags, awkward full suss frame designs etc. the ability to fit a short, fat 500ml or 750ml bottle in a smaller (vertical) space would be a pretty good idea, no.?

Thanks all, good point about the bag...I don't suppose I will need that much space every ride so will use a saddle bag and get a couple of straps to put tube/pump on frame - I'll be able to use the 800ml bottle then.

I'll look at the adaptors and a side opening cage for when I need the bigger bag.

Thanks again

Put the bottle in the frame bag? 🙂

I'm pretty sure Wildcat Gear do frame bags with bits cutout to accommodate bottles.

I don’t suppose I will need that much space every ride so will use a saddle bag and get a couple of straps to put tube/pump on frame

I use a pretty small saddlebag, it takes a tube, tyre levers, tubeless repair kit and a multitool. My mini pump is on the frame.

Drill and add 2 more cage mount bottle bosses