My longer bicycle rides are easier when I have plenty of water to drink. The bosses for a bottle holder on the seat tube are obstructed by a special tool pouch. I decided to make a bottle mount to hang from the seat rails. You can see the finished product in Step 14.
Parts required are a bottle holder, a short piece of 1 x 2 inch pine, two metal or wood screws about 1 inch long, a 1/4 x 2 inch screw, a self-locking nut with optional washer, and two flat pieces of steel or aluminum or PVC.
Step 1: Saw PVC
I chose to use PVC because I have more of it available than I have of suitable steel or aluminum. Saw a piece to the length for the widest distance between the seat rails.
Step 2: Cut Two Rectangles
Cut two rectangles from the PVC. Size them to fit between the bottom of the seat and the bend where the rails begin to run parallel to the top of the seat. Soften the pieces with a heat gun. I place the PVC on a brick to avoid setting the wood on my workbench on fire.
Step 3: Flatten the PVC Pieces
I press the softened PVC between a piece of wood and a brick. Hold until the PVC is firm again.
Step 4: Mark One of the Rectangles for the Rails
I wanted to grind grooves into the PVC so it does not move after the bottle holder is mounted and assembled.
Step 5: Cut the Grooves
I used a burr tool in my Dremel to cut the grooves by hand.
Step 6: Drill the First Hole
I wanted to be certain the holes in the pieces align perfectly. Place the piece of PVC that will be under the rails onto the rails. Align and drill a 1/4 inch hole in its exact center.
Step 7: Drill the Second Hole
Slide the piece of PVC that will be above the rails into place. The grooves should pop onto the rails. Place the piece drilled in the previous step and use the hole as a guide to drill a hole in the second piece. The two holes will align perfectly.
Step 8: Cut the 1 X 2 and Inlet
Cut a piece of 1 x 2 inch pine to 3 1/2 inches in length. Mark the halfway point of its length. Align the hole in the PVC with this mark. Mark the 1 x 2 and inlet the side shown at the right to the thickness of the PVC. (The other cutout notch will be described later.) I cut two of the corners at 45 degrees to clear the seat and for a better appearance.
Step 9: Drill Another Hole
Using the hole in the PVC as a guide, drill a 1/4 inch hole through the 1 x 2. After the next step is finished, you can add a little hot glue if the PVC fits loosely in the notch made for it.
Step 10: Cut a Notch for the Nut
The nut that holds the whole assembly together needs its space to avoid conflict with the water bottle. In addition, the longest screw I had was only 2 inches. I cut a notch at the minimal workable depth. I did not want to overly weaken what was left of the 1 x 2.
Step 11: Fitting the First Piece Inside the Rails
Put the screw through the piece of PVC that fits above the rails. Drop it between the rails and turn a quarter of a turn.
Step 12: Paint the 1 X 2 and Assemble the Parts
I painted the 1 x 2 pine black. Add the second piece of PVC and the 1 x 2. Hold the screw head with an offset screwdriver. Turn the locking nut until the parts fit together snugly, but do not overtighten. A flat washer could be used under the nut. Hold the bottle holder over the 1 x 2. Mark the screw holes and drill.
Step 13: Attach the Bottle Holder to the Assembly
Insert the screws through the bottle holder and into the 1 x 2.
Step 14: Ready to Use
Here you see the water bottle holder mounted and ready to use. The PVC may flex a little, especially when you hit a good bump, but it stays where it belongs. When I drink, I use the bottle mounted on the downtube. When it is empty, I reach back and pull the bottle from under the seat and switch them. This extra bottle has made a big difference in how well I am able to finish a longer ride.