The RhinoCAD design.

The obligatory test cut on the blue foam.

Cutting the L-shape. Since this is the underside, the finish isn't too important.

Cutting the slots. This is actually a second pass (right to left) to trim some of the burrs.

Polypropylene isn't the easiest material to work with, but I wanted to use it to get more comfortable with it and because I have a huge pile of remnants, which conveniently included a piece exactly the size I needed. The swarf has a nasty tendency to fuse and jam the workpiece. It also wraps around the end mill frequently.

All done. The "peach fuzz" left over was later easily removed through a quick once-over with a torch.

And here it is mounted.