This bottle-cage-holder from Profile Design attaches up to two large drinking bottles safe and secure where they do not interfere with the aerodynamics and are still easy to reach, directly on the saddle rails. Simply screw the crossbar onto the rails which are up to 7 mm thick, with two screws (rail ideally made of metal), adjust the inclination and height accordingly - this is easy to do with the slide - and then tighten it firmly. It is best to repeat the tightening process again after half an hour of riding and you're good to go.
Nowhere are the leverage forces for 750 ml bottles greater than behind the saddle and I have lost rows of bottles and similar holders have simply broken off while riding. This holder with its two "vise cages" has never failed here, they really grip like a vice.
It takes some practice and strength to pull the bottle out and put it back in, so please practice before the competition. A little tip: the more angled the bottles are, the better the aerodynamics are.
I specifically include TWO of these cages made from a nylon/fiberglass mix, which puts the initially rather high price into perspective. Of course, you can also just mount one bottle holder in the middle and/or fill one of the two holders with an emergency kit in bottle form, such as a spare tube, cartridge, tire lever, etc..
The weight (without bottles) is about 150 grams despite the strong aluminum clamp, steel screws and two Vise cages.
If you are looking for the right aero bottle for your frame triangle, you will find it here.
Made in Taiwan.