On my Utopia Kranich cross frame the single waterbottle space is given over to the permanently bolted-on carrier frame for the huge, heavy 8.8Ah Panasonic battery bottle; it unlocks from the carrier frame by key, leaving the frame behind.
After spending quite a bit of time and money on various schemes for mounting extra water bottles, all of which proved inefficient, inconvenient, useless, tacky or otherwise unsatisfactory, in desperation I tried one I several times dismissed as obviously a gimmick. Years and years later I still use it. I tell you this long story so you don't dismiss it out of hand. I use mine on the seat tube where it falls readily to hand, but you can use it on any convenient tube because it adjusts to tube diameter. This is it, from the SKS company, who are on my shortlist for beatification the next time there's a cycling Pope (SKS also makes the best floor pump, the Rennkompressor):
It's the strap with the two stripes; the carbon bottle holder is from BBB. The strap has a quick release you just won't believe will work, but when correctly set up it clamps like a limpet. This is what it looks like bare.
The SKS part number is 10505.
A full five star recommended component.