I have one of these and it works well. The base is magnetic and there is a v-shaped groove so it sits on a round tube properly.
It can also tell you the difference between two angles, in case you r top tube isn't level, e.g.
Amazon, Lowes, Home Despot all have them.
Klein angle finder
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72 Fuji Finest 76 Super Course, 72 Gitane Tours de France, 74 P-10 & 79 Tandem Paramounts, 75 Grand Record, 76 Grand Jubile, 84 Voyageur SP, 85 Miyata Sport 10 mixte 89 Cannondale ST400 and a queue