I'm just finishing up the border today and should be grouting later this week. Here are some photos that show some of the details.
Here you can see the long cleat on the back of the mosaic, near the top. The bottom of the cleat is 10" from the top edge.
Overall shot. Just the border left! Sides and top and bottom are about 69 1/2" inches. (Some spots vary slightly down to around 69 1/4".)
The cleat under the top. (The screws ends will be cut off so they are nearly flush with the nuts.)
Cross section. You can see how the glass is slightly uneven in thickness. Including the cleat depth, the thickness is about 1 1/4".
The frame (better than this block of wood!) will cover part of the glass edge and finish it off. About 1/4" or 5/16" overhang on frame might be good.
The frame profile might look something like this.
The top and bottom cleat that will go on the wall.
(Vicki: Dusty Bill 24" x 24". Last original from The Girls and Boys of Mother Goose. Unframed.)