Did you solve this? I am making a plan for my I-28s similar issue.
I have started part of my solution to this problem. I have attached several 3/4 inch thick plywood blocks in locations I want to space the FRP material away from the boat hull. I will use the plywood stand offs to also secure the mahogany batons running port to starboard. I plan to use mahogany plywood to cover the funky shaped areas above the galley and 1/4berth, making the headliner just straight. I will attach several plywood blocks to attach the mahogany plywood to. I think it can be done with two flat pieces to close off the area, then put small mahogany trim for a corner, and 1/4 round where it would meet the old plywood paneling.
I'm going to attach some trim where the headliner was stapled to hold the edges of the FRP. Where there is the funky plastic trim, I will just use mahogany trim instead. I may have to glue up some thin wood strips to attach the trim to. It will look great but take some effort, the material is very affordable.
You can get the FRP in several colors, smooth finish or Ice finish. Home Depot sells and stocks 4x8 pieces in white with the Ice finish. I plan to use the smooth finish, more of a mat finish, less reflective, just my preference.
Here is the stocked FRP at Home Depot.
4 ft. x 8 ft. White .090 FRP Wall Board-MFTF12IXA480009600 at The Home Depot
You can get the mahogny plywood and trim pieces at a lumber yard that supplies hardwood and cabinetry wood trimming.