This frame is designed to be used with 4" embroidery hoops. 

There is wiggle room where the hoop is inserted. I used an adhesive weather seal tape (for windows) to create a snug fit. The gap can also be filled with extra fabric, hot glue, cardboard, or whatever your crafty heart finds. I chose to not make it a tight fit because different fabrics with embroidery have different thicknesses.

The holes on the back can be used for magnets. I added a hole in the holes up at the top so that it can be strung for hanging as well.