Interior dimensions : nearly all of them are 9-15/16" tall by 7-15/16" wide. You can fudge the depth by how far the inner "tray" sticks out. I had some pretty deep Super Powers figures in them simply by taping the sides to the plastic. I do have about 20 larger ones that I had Secret Wars figures in (and then Super Powers coloring books & stuff) and those are 11-1/16" tall by 8-9/16" wide as far as interior dimensions.
2. Who produces/manufactures them?
Half are Walmart's brand (before they discontinued them), the other half are from Michael's. They retail for $2.99 & up depending on your location in the US.
3. (the dumb question) the white cardboard flips to encase the fig, right?
On nearly all of them I've already cut the back/flipped the cardboard/taped the corners, so the work is already done. Some I had proof cards in so they haven't been "flipped" yet and the cardboard insert is still "solid" (if you take my meaning).
I also snapped this pic just in case anyone wonders if the wire easel stands I'm selling will support a slabbed comic. The book is about 12" by 12" and is a 400-page hardcover weighing more than a couple pounds.