OK, here's a very quick sketch of a jig for a handheld router. Hopefully the different views will make sense.
Basically it consists of a long sub base attached to the router. The sub base has a guide attached to it. This guide would be the same width as the diameter of the bit. It would be spaced the same distance from the bit. In other words, if the bit is 1/4"Dia. the guide would be 1/4" wide and there would be a 1/4" gap between the bit and the guide. The router bit would extend to the thickness of the work.
The sub base would ride on two guide rails which would beslightly thicker than the work. The guide rails are attached to one of the sides of the jig. These can be clamped together along with the work in a vise or Workmate or whatever.
The next view shows the clamp open and one side of the jig moved out of the way.
The work would be clamped into the jig with the boards staggered by the diameter of the bit. Although I only show 2 boards in the jig, all four could be clamped up to allow two corners to be cut in one set up.
=the first cut is made with the guide fence on the sub base against the Guide rail on the clamp. The succeding cuts would be made with the guide fence riding through the prvious cut.
It occurred to me that one might clamp a piece of scrap on the out feed side of the jig. It would get a groove cut in it allowing the guide fence to be controlled as you make the next cut.
This was very quick and maybe not as clear as it could be. Maybe it will give you an idea or two, though.
Dave