With the initial mounting holes for the flap houding drilled I used a couple #8 screw backmarkers to match up the new cover part with the existing screw holes. I still need to add a couple more screws, but the installation is mostly done. I put the seat back in place and took it for a test-sit. What a night and day difference. The interior is much more comfortable now. Once I move the seat bottom back 1" I will gain some more legroom also.
I did some testing to see if I could paint the new parts by brush so I wouldn't have to mask off the interior again. That didn't go well at all. I will have to touch up the paint the hard way...