While I'm waiting for my boards to show up, I put in a DigiKey order and thought about how to power this rig. The pack of NMH batteries used by the original hardware wouldn't exactly cut it even if they hadn't leaked everywhere. Some of my designs use the MCP73871, which is fairly easy to use and works great for lower power systems, but doesn't support the kind of amperage an entire notebook computer will need. Ideally I would roll another design with a stronger output, but I just don't have the mental bandwidth for that right now. This has been a sticking point with a lot of portable designs in the past, but fortunately, a number of commercially available options have cropped up that should meet my needs.
As an IT guy, I have the typical distinction of being the person friends and family call with computer related questions and issues. This has lead to the refurbishment of various laptops: SSD/RAM upgrades, screen and battery replacements, etc. If you're reading this, you know I kept those old batteries and salvaged the 18650 cells. My most recent leftover had four cells in it, two of which were still viable. I can't guarantee how they'll perform, but it would be nice to put them to use.
Looking around online, I ran across this design: amazon.com/gp/product/B082CVWH3R . A three amp output should have enough headroom for the Pi 4, the screen, and some peripherals. Hopefully it performs well. I also hope those used batteries are up to the task, but if not, I should have some new ones somewhere in the parts heap. At least the board fits into the case well.
It occupies a fair amount less space than the original battery pack. I haven't decided how I'm going to integrate the power button and battery state outputs into the design, but one thing at a time. I'm also not using the pogo pins intended for attaching directly to the Pi, so it needs some decent standoffs (probably a 3D printed frame) to keep them floating above the bottom of the case. I've covered the battery area with kapton tape to reduce the possibility of shorts, and because battery leakage abatement is an endless fight that I'm choosing to literally gloss over for now.
The inside of the case is still pretty ugly, but there's only so much I can do about that. It should be covered before too long anyways!