As part of my planning around a DIY FreeBSD laptop, I discovered that someone had already built more or less what I was thinking of - an Intel SBC inside a Pelican case with battery, display, etc.:
This really is a great build, and I found it quite inspirational. Especially because it confirms that my ideas are (roughly) sound and feasible.
You might like this subreddit: https://www.reddit.com/r/cyberDeck/