Here’s what you’ll need:
Sheet Metal- available at 1st Coast Metal Roofing Supply
Once you have all the materials gathered, first you’ll want to measure the part of the frame that the metal will fit into. Draw measurements onto sheet metal with Sharpie. Cut your sheet with a pair of tin snips along the Sharpie marks. Place the cardboard that came along with the picture frame against the sheet metal, add your frame backing- and you’re done!
A little side step to this project would be- to order a piece of sheet metal already the size of the frame.