If the problem truly is to measure thrust angle, Phil's approach is a good one. If the problem is more precisely to set up a recommended thrust offset--let's say instructions call for 3 degrees of right thrust--then I use my drafting machine and just draw the lines of interest: thrust line, firewall squared to thrust line, firewall offset 3 degrees to right, distance from firewall surface to mounted engine center of prop hub, offset of engine mount holes on slanted firewall to keep center of prop hub on zero thrust line. All this can be quickly drawn at full scale. Then I drill holes for engine mount (I normally use a dubro type mount), mount the engine, and assume my holes are close enough to give me three degrees. I see no point to actually measuring the result, since the acid test is to fly the plane and see what it does in a slow climb. If I'm off one way or the other, then it's time for shims and washers.