Is the planking more than what primer will handle? It's hard to see in the pics.

Oh, and definitely fiberglass resin for the edges and corners. It flows well so it'll fill in the whole area that you dam off. It worked better than i thought it would on my whirlwind. I was thoroughly impressed. The stuff stinks though.

It's actually extremely easy. Dam off the corner with something rigid, pour in the resin, wait a few hours then sand.

The resin flows better. Here's an edge like one of yours.
IMG_20190808_205558 (resized).jpg

Yeah, a piece of flashing would work. So would plexiglas. Just put a coat of wax on whatever you use so the resin doesn't stick. It makes a huge difference. I just happened to have metal scraps sitting around in my shop.

Sand down a portion and see what it looks like under the paint. You're going to repaint the entire thing anyways.

If you use tape, just make sure that it doesn't cave in slightly. Make it bow out and then sand the edges sharp.

Don't over think it. You can "what if" yourself to death. You've got a good plan. Follow it and you'll be fine. If issues pop up along the way, you just fix them. There's nothing that you can do to that backbox that can't be fixed.

Just don't start it on fire.