I use factory edges for a piece of cardboard, after checking to make sure there is a square corner. One side goes against the plastic right edge of the crumb tray and the other butts against the front drop down door. I’ve also used a squared piece of cardboard and registered it against the edges of the garage door. The crumb tray and front door is easiest.
Then I cut out the areas where the pencils go or the clothespins, or the tongue depressors. I have also kept the same frame and then use a replaceable center rectange that fits with whatever frame jig I use.