In one of your other posts, I see that you have your rear glass out. The trim will cover up SOME edge irregularities(That is part of its puropse, to allow flexability in manufature and in real life movement) . That will make it MUCH easier to make a template from craft paper or corrugated cardboard. You said you were going to paint interior stuff, so I ASSUME the rear window trim is among these items. Go to a Hobby Lobby, or other 'craft' store, and get some of the heavy craft board, or thin fiber-board, to make a new rear package shelf base. You can PROBABLY get some vinyl or cloth to cover it there also but it most likely will NOT be automotive grade (conforms to FMVSS); If not try an Automotive trim shop (place that recovers seats, does convertible tops, and/or custom interiors) and purchase some material from them , or have them make the package shelf for you. Their materials should meet said specs. Regular household and 'craft' materials do not have to, and WILL fade a lot faster; they are just NOT designed to be exposed to outside all the time... If it has slots or vents or speaker cut-outs, use a router. If it was just painted cardboard, then PRIME and PAINT it...(Either the cardboard or fiberboard will soak up a LOT of primer). Unfortunately, you WILL have to get creative... You are doing a restoration on a not-too-popular car (Buick Le Sabre), made in a not-too popular years (73-77) among the restoration crowd. If the major restoration crowd doesn't cover the CHEVY's for that period, figure the odds of them covering a BUICK! There just is NOT that much out there for resources, unfortunately... Sometimes you just have to make your own parts as best as possible... At least it is not sheet metal!
You can find the correct paper material at upholstery supply stores. You will have to buy a sheet and cut it to shape. If you want, you can paint it the color you want. Or, you can put vinyl on it to match your upholstery. You can also get the vinyl material that glues to the metal behind the seat for a neater appearance. The guys at the upholstery supply can steer you right, and if you don't want to deal with too much of it, can point you to a shop. Make sure to get other referrals for the shop. I had a lousy experience with an upholstery shop, on my Buick back around 2001, even though they were recommended. Had my dad been alive, it would have been done right, that's what he did.
