The bevel will be done by sawing the diagonal of a square of 15 x 15 mm.
The photo frame 13 x 18 is about 5 mm thick and it's a glass-mounted photo. (picture : see below)
The notch in the wings of U section will be 6 mm and very little tilted.
But you can also make a 5 mm right notch and to finish the slope with the file.
The holes will be drilled in the inner wings of section : be careful the direction !
You can choose threaded rods and nuts which are cadnium-coated : they have the advantage of not rusting, do not forget the 7 mm wrench !
Options :
For a 18 x 24 glass-mounted photo, the diagram and the advice are the same ones, except the aluminium tubes length is 100 mm and the threaded rods length is 115 mm.