Thanks guys. Starting to look like something...
Past the scary part... I used the edge of the ABS circle to cut the new circle out of the MDF. I thought about using a router, but haven't used one in years... didn't want to chance it. Forgot how much fine sawdust MDF makes. Got it cut out and drum-sanded smooth and even with the ABS edge. Did a test fit over the lower port. Also taped in a piece of edging that I'm thinking of putting around the inside edge for a nice uniform edge. Epoxying the ABS to the MDF, final sanding, then a test fit for the red port.