Get a good one. I use a Slope View TLL-90 (Dual-Axis Digital Protractor)
I milled pockets for stronger earth magnets. You can get them off ebay. Rechargeable too.
Angle cubes and the like are rubbish in comparison. It a bit thicker than a wicked edge hone, 3/4 the length and double the height. It’s back lit and you can even set it up to beep if it is passed the angle you desire. Got a lazer pointer too.