I use a DeWalt DW611 Router as a spindle on my Shapeoko 3, and also use a Super-PID controller to control the speed of the router. The Super-PID controller requires an IR Tachometer to be mounted in such a way to see the spindle, which has to be painted 1/2 white to provide the rotational rate. There are many ways to mount the probe, this is one of them.
After rewiring the internals of the router to be used by the Super-PID, there is a large cavity where the power switch used to reside which provides easy access to the top of the spindle. In the first picture you can see the spindle painted white. This piece fits snugly into the cavity forming a friction-fit, and allows the IR probe to be inserted to see the painted spindle. A dab of hot glue affixes the probe at the correct distance from the spindle (~2mm.)