NavList:

Hello Joe

Flat sheets of any material, with parallel surfaces, merely change the displacement, not the angle.  Since we are measuring angles, there is no effect.

Be careful of that statement.  How do you know the surfaces of your plexiglass are flat?  How do you know they are parallel?  Glass has those characteristics due to the method of manufacture.  How is plexiglass manufactured.  Davis uses small sheets well supported by the AH.  That means deflection under self load are minimized, minimizing any angular change.  How do you intend to support your sheets, minimizing this deflection.

The Davis AH is not a top end device.  It is specifically advertised as a device to practice with.  Small angular changes due to non parallel, non flat surfaces can be waived off, since it is just for "practice".  Any time you see the words "for practice" in an advertisement, you can be sure that the device suffers from some loss of accuracy due to something inherent in the device they are advertising.  Otherwise, they wouldn't state such a limitation.

There are plenty of homemade mirror devices described in the archives.  They aren't affected by wind.  But they may leave you in doubt as to the accuracy.

Of course, you could just purchase a Freiberger Artifical Horizon.  Ready made, with stellar credentials 

Brad