Scrap piece of thin wood
Plastic tape "Bearing"
scrap of thin rubber mat/cloth
1) First cut the plastic tape bearing in half, this is what will increase the diameter of the handlebars so the watch part of the heart rate monitor will fit around the smaller diameter tubing.
2) Attach the two halves of the tape bearing to the handlebars by wraping electrical tape around them.
3) Cut the wood to the size of the bottom of the heart rate monitor, this is what will keep the monitor facing you while riding.
4) Cut the strip of rubber mat/cloth to the size of the wood, this isn't entirely necessary but it helps keep the monitor a bit more snug.
5) Using electrical tape, tightly wrap both the rubber/cloth and the wood to the tape bearing at an angle slightly towards the back end of the bike.
6) Ride off into the sunset, while knowing your heart rate the entire time