I was thinking about drFrost's 2.5mm charger solution that charges the battery directly, so no need for micro-soldering, just two loose contacts to connect to dual battery mod.
Might I suggest rather than an acid dip, actually anodizing the aluminum? This would give it a very tough finish that would really add to the look (and prevent oxidation to boot). As a nice side-effect, you can provide a range of colors (based on what your local anodizer shop can do). Gloss black would be nice, but so would an intense, brilliant blue. As to the texturing, I'm guessing that by sanding, you're going for the "brushed aluminum" or "spun aluminum) texture. Another possibility for a nice grippy texture is a quick hit with a course sandblaster. Also, for something like this that it sounds like you're going to be milling very thin (to keep it light), I'd suggest using "aircraft" aluminum - 7075-T6. IIRC, it's roughly double the tensile-strength of "marine" aluminum (6063-T6). I miss working in a machine-shop... it's been over 2 decades. I, too, am very interested in your homebuilt mill. I hope you took lots of pictures and video of the build process!