Should I add extra bottle mounts?
 

Can I add my own water bottle mounts to an cannondale cad3 frame? I'm looking to support a load of around 15 pounds between the 4 existing mounts, but am unsure if it would stress the frame at all..

15lbs is way too much for a rivnut in thin aluminum IMO

15/4 pounds too?

A large waterbottle weighs 28oz and is supported by 2 bottle bosses. A single boss supporting 60oz is probably pushing it; the result, a ripped out rivnut, would undoubtedly include distortion and fatigue of a pretty thin tube.

Let me put it to you this way: I sure wouldn't ride it. A steel frame made from 9.6.9 tubing could probably handle it.

I think I will skip going into the frame and instead build a pack around the frame with straps and clamps in lots of places. My bianchi tsx is like... 7.5.7

What is it that you're trying to carry?

Battery pack. It would rest in lower corner and occupy about 1/4 of area between all the tubes.