I guess it depends on the specifics of your bike. I addition to whereever you put one on the bike maybe one on the helmet or other wearable lights?

Maybe the seat stays is the general next best after a seat post? Something like a fibre flare would work well on a stay. You'd need to think about if you need one on either side and that they won't, eventually, swing into the wheel.

You can also get creative with Sugru to make your own custom fit spacer

Maybe a picture of the back of the bike might help suggestions?

Bontrager circuit mips helmet has a magnet on the back where their flare light, I believe, fits onto. I bought it, but because I’ll be in aero position for most of my rides, the light would be pointing up, so I bought a seatpost mount for it, which still lets me use a bts system for water bottles

Lots of options that can clip to a pocket, collar, helmet...Also rear non drive side seat stay can be a good option.

Is there no seat post visible below hydration? My saddle height is 70cm on tri-bike and there's plenty of room beneath the bottles.

Serfas uses rubber band type thingies to clip to the seat seat. $30ish bucks, recharges on USB, and has multiple modes.

My rear hydration system has a pump adaptor. My bike light fits perfectly onto the pump. You could probably do the same thing with a CO2 adaptor.