Step 1: The Original Reaction Timer
Step 2: New Reaction Timer
Step 3: The base
The length and angle of incline are up to you. The original version used a switch to detect when the golf ball reached the end. My version uses a IR LED and detector for the same function. The golf ball blocks the IR emitted by the LED from reaching the detector when rests at the end of the rail.
Step 4: The Underside
Step 5: Microcontroller & Batteries
If there is significant interest in the project, I'll post the schematic and source code as well.