When working on my sys port, I turned my Domino around 180 degrees from your picture. To get a reference surface, I made an L shaped jig out of two pieces of the same plywood I was using for the sys port (avoid thickness issues).
I clamped the L jig tightly to the end of the sheet with one leg going under the sheet to be cut and the other leg going up at the end of the sheet to be cut. I thus had a reference surface several inches tall which I used with the base of the Domino. Curse the choice not to take photos at the time. Now due to recent neck surgery I cannot recreate the set up I used for another 6-8 weeks.
The vertical pieces could then be cut/dominoed be laying them flat on my MFT.
My plan is to make two sys ports and document the methods on the second one when I have most of the mistakes worked out in the first one.
If this does not make sense, I can try to do a diagram later and post that.