I used the B-Rad system
To mod my Trek 720 to carry five bottles on tour this summer. One was on the underside of the down tube to lower the bottle, allowing a large bottle without tire rub. One was on the inside of the down tube, putting one bottle in somewhat the normal place, and above it a double adapter so two could be carried side by side. The third, on the seat tube, was used to raise the bottle so as not to interfere with the lower bottle on the down tube. Rock solid and effective.