It was really easy to make - if you have the right cutter. The cutters are called Keyseat cutters or T-Slot Milling Cutters.
Just cut a .25 slot with a regular mill bit then with x and y positioned the same, change to the Keyseat cutter and position it at the bottom of the slot and cut away. There was quite a bit of built up of shavings in front of the cutter so I backed up every couple of inches and cleaned out the slot. Then I turned the stock around and repeated for the other slot.