Foamboard Edge Bracket (Stackable with toothpick reinforcing holes!)

thingiverse

Print as many of these as you want for the dimensions of your enclosure, stacking them together joined by toothpicks through the hole running the length of the clip to fill the seam of your enclosure. Then tap a toothpick into the holes in the back, piercing and expanding the foam board in the clip to lock it in far stronger than the plastic alone. Break off the exposed side of the toothpick flush with the surface when its set.