In that same area right behind the body door pillar, flowing into the flat panel beneath the window across the back my head liner panel forms a smooth curve at the top, but I have a seat belt retractor at the bottom that pushes me to a formed corner. I have a luan plywood flat panel across the back, secured with industrial velcro, but I've been pulling my hair out for two days on these curved filler panels, trying to keep them removable for access to a wiring and door light switches, but secure for upholstery. The curve is too tight for my nominal 3/32" FRP panel to bend easily, and package tray "mat board" material doesn't like the sharp corner at the bottom. 25" tall, ~18" right angle sweep at the top, and ~12" right angle at the bottom. Any thoughts on panel material for this filler panel?