Update: The aluminum angles that hold the mirror centered flex a little bit when the optical tube is pointed at the horizon (as in it's stored or parked condition).  I have twice had the optical tube go below level and found the mirror had slipped at the top a little bit as a gap occurs from the flexing below.  It's small movement, and it's not like the mirror could fall out; the diagnonal dimension of the mirror is a lot more than the tiny gap that occurs from flexing.  But it did allow the top triangle support under the mirror to slip from it's center pivot.  So I added three mirror clips.  They are beefy and are nutted top and bottom at the end of 1/4-20 all thread.  The all thread passes through a plate with a spring and a nut on the other side.  I can push up on the all thread and turn the clips out of the way if they really bother me, and only swing them into position when parking the telescope.  I'll get pictures next time the mirror cell is out.  I finished the job right at dark and wanted to get a view.