Under down tube cage- which one, if at all.

Reported for being an idiot

the only thing i ever stick in my under downtube bosses is the MSR fuel bottle when im on tour. As you point out anything else gets barkit in spray off the wheel. 

Yeah, got a bottle cage in the same place on my bike (the only bottle cage on it). Never used it as it would be covered in all sorts of mud/slime/poo whatever.

Mate of mine carries one of those tool bottles down there with an extra securing strap, seems reasonable to me as they seal up and you're not often into them. 

I keep a fuel bottle there that would get covered in sheep shit/road filth if I didn't wrap it up in a poly bag, which works ok, but might not be the look you are after for your new bike?

I've got a bottle cage there on my bike. Just a cheap bog standard one made from aluminium wire.

I use a large Elite bottle with a cap cover and a voile strap to hold it in place. It's worked fine although I usually decant the contents into the other easier to access bottle for drinking on the move. The cap keeps the mud and sheep poo off well. Not had the bottle jump out yet. Done a fair few rides with it last year including Traws Eryri so don't see it being a problem on a road bike.

https://www.elite-it.com/en/water-bottles/sport/jet-plus

PS not just used for touring but whenever I need 2 bottles as I can't fit 2 bottles and a frame bag in.

I normally carry a tool keg for in a regular bottle cage under the down tube when bikepacking but have also used it to carry an extra water bottle if needed for cooking when camping.

Tool bottle when doing multiday trips. It's stuff I'm hoping not to use but want to bring 'just in case'.

I Just have an Aluminium cage under there (ou can bend an Ally cage to grip a bit tighter if needed see). I can put a tool keg in it (doesn´t have to hold tools of course, you could stuff a packable jacket in it I suppose) or a 1L bottle if I need lots of water that day.

As for getting sprayed with muck, well you can just empty it´s contents into your other bottle, you´ve got to stop to access it anyway (unless you are waaaay along the ape index) I don´t really see the problem other have with this. 

But yeah, it´s just a third bottle cage, put a bottle in it (if you need lots of water) or leave it empty an no harm will befall you. 

Stowage wise, it´s a narrow spot between spinning crank arms, rings and the front tyre, you´ll struggle to get anything much bigger than a Large water bottle in the space TBH. 

Of course if you´ve got bosses on the fork legs too, that way lies madness; before you know it you´ll have a bike covered in luggage!

Regular cage of your choosing, some kind of strap (voile, velcro, whatever). I've often ended up with my second bottle there when bag arrangements mean I can't get a large bottle in main frame (eg, I'll have a bottle and tool thing in cages one and two, another bottle in three).

Yes, it gets covered in crap sometimes, but all my bottles have fliptop muck covers (like fidlocks do, like the Elite bottle above does). All my bottles are like that, there's enough muck on the world, and it's a non negotiable for anything under the down tube.

I have a Fidlock cage there on my gravel bike so if not using it there's nothing to potentially catch on anything (less of a consideration I guess on a road bike), looks a bit better to IMO if not in use. I use either a Fidlock bottle or Fidlock universal bottle mount + tool bottle (can get waterproof ones). Also have one of their Essential bags to give more space as an option but haven't tried it yet.

Fidlock is a neat idea. I’ll check out the options.

i guess it’s that or a king cage Anything Mini.

or whatever it’s called

Something like this maybe - but I'd avoid anything I drink out of being a target for sheepshit

https://www.bergfreunde.eu/fidlock-toolbox-bike-base-bike-bag/