I picked up one of the digital angle gauges from Lee Valley last Thursday.
It comes with rare earth magnets both sides and is solid and relatively
heavy. It can be set to relative zero and fully recalibrated if for any
reason that it loses it's settings. It runs on a 9v battery, so you wouldn't
have to worry about being gouged for some expensive little watch type
battery should you need a new one.

My only complaint is that the magnets don't hold as well as they could the
closer you get to a vertical surface, but it's situation one is not likely
to encounter very often.

LV also sells a non-digital compass type version of an angle gauge, but
considering the cost difference is only $11, the digital one gets my
preferred vote. Another toy to add to the pile.

http://www.leevalley.com/wood/page.a...at=1,240,41064