It's been bugging me since I did the headliner. The canvas I bought said it was "60 inches" wide. The cloth was actually 58" wide, so I had a gap of about 3/8" at each edge on both sides where the ceiling meets the walls, the entire length of the cabin.
Made a couple of flat trim pieces to do the edges, stained, finished & installed them. Here's the back at the cabinets above the port holes.

This is the front at the headboard, where the curve starts at the front radius.

Will be working on the air conditioning this weekend, just in time for summer (actually it's simmer here now).