Making a nice gap on something with a radius, like a bumper. First, gaps always look better if you use something more rigid, like acrylic. It stays more consistent, straighter, and looks much crisper. Generally speaking (there's always some exception), start with the straight sections first, to establish where the gap will exactly be. The first 2 pictures are using plastic to gap the straight sections. The remaining section is the round corner. For this I used a piece of flat rubber to make the radius, it is soft enough to easily make the curve, but rigid enough that it still holds its shape. I used a micrometer to make sure it's the same exact thickness as the plastic I used. I put the rubber in the corner, squeezed some filler into the corner and let it dry. Once the filler was dry, remove the rubber and sand everything flush.