These are router templates for turning square corners into round corners. They work with with flush trim router bits* and a router table (3d print coming soon). I usually tape them in place to prevent them from shifting while cutting.
I uploaded a few different options, but you can customize your own with Onshape. There are three starting presets (see tabs at the bottom), one for circular fillets, another for conic rho fillets, and another using splines.
-- Atomic Filament Clearance Transparent Red PLA
-- Atomic Filament Clearance Starry Night PLA