Have you tried doing a inclinometer search on Amazon. There are several digital incline meter. Problem is that the readout might be facing the wrong way depending how they read and how you mount it.

A cheaper meter would be a surface bubble Apps if you have an Iphone or a phone that can download apps. I have a Clinometer app on my Iphone that I can recalibrate to zero after mounting my Iphone to my handlebar. It can read it degree or slope and cost less than a dollar for the app.