aluminum adjustable router template

[Dodis]

Here is one example of a homemade version...

https://www.youtube.com/watch?v=InipG5bb7B4

Mike "Dodis"

[JGrout]

At least the one I posted can do angles 

[Routerman]

Might be that a precise & flat base plate will facilitate routing the pockets (given, they're rectangles).
A round plate's diameter is rarely centered on the cutter. But a rectangular plate, along any edge,
will yield a straight cut line (in a mortise), especially on inside templet cuts. Of course, for every cutter and edge, the
thing has to be calibrated. I would rather do this with a collar/bush; your chances are better and the complications/calculations are fewer.