I reused a mylar mirror from an old rear projection tv:

(Picture angle makes it look odd; it's hard to photograph a mirror.  It is trapezoidal shaped)

Mount

This mirror didn't have sufficient mounting points, so I made a couple brackets to hold the mirror, and allow me to hang it.

I lined the brackets with PVA foam where it contacted the mirror frame.

On one end I attached an eye board for hanging(though I would later move this position), and the other fashioned a latch to allow the mirror to be stowed flat against the ceiling.

Pieces were attached with machine screws and tapped holes for easy adjustment.

Two hooks were screwed into the ceiling for the eye bolts to hang from.  The other end was suspended from some cord.  I added some very lightwieght elastic which would pull the cords back on top fo the mirror back when it was lifted to the stowed position.  (the black line below, which was attached to the cords with some thread which doesn't show in the image)

To making fine adjustments of the mirror angle, and twist, I used locking collar's on the cord, locked in with a set screw.  (these are RC linkage stoppers)

To stow the mirror, flat against the ceiling, it is simply tipped up, and the latches hooked on the ceiling hooks.  The cords automatically are pulled back by the elastic.  To lower, I simply pull back the latches and lower it down.