You don't tighten anything. The mirror rests on three ball bearings and is held in place by three blobs of silicone glue. In other words:
1. Place your three ball bearings down. (I probably would drill little divets for them to rest in.) Bob actually may have suggested using BBs. You want them to be small. 2. Cover 'em with silicone glue. 3. Place the mirror on top and let dry.
This is similar to the nails that Berry uses as spacers, but you never remove 'em.
Mark