I've got an '04 Cannondale and one bottle cage on the seat tube, but I want to add one on the down tube. There are no mounting points on the down tube, which is somewhat egg-shaped. Any ideas?

You could use one of the holders that mount under your saddle like the tri-geeks use.

Google Zefal Gizmo. They are cool, easy to install and relatively unobstrusive. This is a picture of them on my Falcon, which did not have cage mounts on the seat tube.

Your post says there are mounting points on the downtube? If so, why not use them?

Sorry, post edited.

Zefal makes a universal model of the Gizmo, which may just be perfect for odd sized or shaped tubes.

https://www.zefal.com/en/bottle-cages...universal.html

You can also use hose clamps if you don't care too much about the appearance. That is what we used to do "back in the day" when lots of bikes did not have mounts for cages.

Small calibre pistol. Self tapping screws.

Use the clamps they used to use to attach cages to frames. You might still be able to find bottles, cages and clamps in packages at Walmart.

sounds like his best option and maybe add rubber strips or a few wraps of the proper color electrical tape to protect frame. Alos how does a 2004 bike not have 2 mounts? must be a hybread.

Seriously, hose clamps amd tape, when for the same price you can buy the purpose made Zefal Gizmo?

What model Cannondale does not have downtube bottle mounts ?

Ironman 600 Tri bike. OP, a saddle mount bottle carrier or areo bar bottle carrier may work for you.

I drilled/tapped my own on my first steel Trek. Piece of cake.

https://www.nordicgroup.us/cageboss/​

electrical zip strips work perfectly fine, just use 2 on top flange and 2 on bottom flange. plus they come in colors so you can get close to your frame color.

Two Fish bottle cage.