Bottle on frame alternative I've been riding with a bottle filled with isotonic drink + hydration backpack with plain water for quite awhile. Recently my bottle stated to fall off on descents (not the best frame design + increase in speed). I'm looking for alternatives to frame mounted bottle cage that will suit aggressive trail riding. Solutions like Source dual bladder won't fit, because once you had a sweetened drink in your bladder it's just a matter of time before mold starts to grow there. I've looked on runners belts, but 600ml is way too little for hot conditions. Assuming I want to separate between sweetened electrolyte drink and regular water, and a minimum of 1l liquids all together, what are my options on the market nowadays?
First of all: You mention that the placement of the bottle mount is bad. There are adapters which allow you to mount a bottle cage without frame holes. E.g. SKS Anywhere, Minoura or this set. All look rather flimsy though. Mounts for the handlebar or behind the saddle usually look more sturdy, if you have space there.

There is also the Salsa Anything Cage which allows you to mount and tie down anything anywhere.

If a better, more vertical mounting point for your existing bottle cages doesn’t solve the problem, maybe you should get better bottle cages. For example I use these Elite bottle cages with various normal ~800ml bicycle bottles on my cyclocross and I’ve never lost a bottle (except in a 40km/h crash, but that doesn’t really count). They also seem quite popular.