Indeed (@ datzenmike), if I had those mirrors, sheet metal screws would do, but as I said, many/most of the mirrors available (including the ones I've got) don't have holes in them to screw through from the top; they are made to be bolted from underneath ..
Before I modify or give up, I thought I'd see if someone else has figured this out already.
(Like these: http://r.ebay.com/pRA9YV or https://goo.gl/2uCfl3 (#52163 and 52165) or ... yeah... I'm open to other suggestions)