DI--you can build a fairly accurate accelerometer on the cheap if you are willing to do some math. All that is needed to build one is a piece of stiff wire (e.g. straight piece of coat hanger), a spring with a known spring rate (can be measured if need be), and a weight of known mass (e.g. fish line sinker.
Put sinker on wire, attach spring to weight and end of wire. Can wrap a small piece of string around wire at base of sinker when at rest, anchor contraption on dash, and accelerate. Weight will slide down wire until spring prevents further movement. Measure distance from where the string started to where the weight pushed the string. With this distance, the spring rate, and the mass of the weight you can calculate the G-force exerted during acceleration (break out the old physics books with Newton's laws of motion) I assume there is an internet calculator that can estimate RWHP based on weight and G-force during accelleration, if not it can be derived.
Definitely requires more work on your part, but the calculations could be easily programmed in MS Excel, and the device can be built for $1-$2
If you want to pursue this, I can make the spreadsheet to do the calcs and explain how to find the spring rate if need be.
-S/6