While waiting for the guide bushings and router bit to arrive, I fabricated these side stiffening brackets. The left one (shown here) was made from a piece of scrap angle which had an extra hole. No one will ever notice. ;-)

Guide bushings and router bit arrived.

First test in a piece of scrap (view from below).

That worked swimmingly!

All 26 notches completed. Except for one slot which is slightly wider than intended, they came out great. That was my fault. I tried using spring clamps to hold it to the fence instead of c-clamps.

Here's a close-up view of one of the slots. Very little cleanup and deburring will be required.