A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2019 Jan 7, 20:43 -0800
Is your mirror really only 1 mm (0.039 inch) thick? It is extremely likely that it is distorting. The double sided tape is likely pulling it into the shape of the plastic you have mounted it to.
Some may argue over how thick or how flat the mirror must be to get an optimal sight, but in the end, your result will be related to flatness.
A liquid artificial horizon, such as the fairly inexpensive Davis Instruments AH, would be an easy place to start. I won't argue you out of a Freiberger, that is a quality instrument.