ABS is really heavy and hard to work with compared to PVC foamboard. Once you get the 1/4" luan plywood installed, you make the PVC foamboard 5 or 6" less all the way around the perimeter. Stitch around the perimeter of the PVC, glue the whole assembly to the plywood, and then you can finish the edges off just like a regular headliner. If you need to make some panels for the sail area, so be it. Look at this thread, you may get some ideas:
Custom Headliner for '36 Ford Pickup