I've wee legs so the frame triangle is fairly compact - i.e. a couple of 750ml bottle nearly fills it.
I'm loathe to start strapping bottle cages to my forks, though I do have an extra set of "anything" cage mounts on the underside of the downtube.
I'm currently thinking of;
(a) Cutting down to one in-frame bottle and having a custom-made bag to fill the remaining space. I reckon keeping the seat tube bottle would work best. I might be able to fit a Shimano BA01 battery mount adapter to let me move the cage down a little and hence fit a slightly taller bottle in too.
(b) Getting a custom (or off-the-peg) frame bag and using a water bladder in it. I'd still have the option of using the 3rd bottle cage mounts too.

FWIW the existing cages are side-loaders and work really well.
Any thoughts from the collective?