That's the common setup; the playfield hanger hooks fit over the smaller of the two cross members, and are secured with some sort of clamp to the smaller cross member (some vendors include 2-inch "C" clamps along with their units). You'll use 3-4 of the brackets with the thumb screws for the other end of the playfield; you can use 3/8- or 1/2-inch long hex head screws through the slotted holes in the brackets into the bottom side of the playfield so the playfield can be safely rotated. (Pick the length that when coupled with the thickness of the bracket won't exceed the thickness of the playfield so the screw won't pop through the top side!)

Because the playfield hanger hooks are too small to fit over a 3/4" wide channel? And using a 1/2" wide channel at the "top" end of the playfield would probably allow more rotation (downward sag) of the brackets?