I'm insanely happy with the results of our first prototype, StereoBoy R1.

Pretty much all hardware functionality is confirmed working, from small things like the thumbwheel potentiometer to XIP from QSPI flash wired over a GameBoy cartridge connector, which I was pretty scared on whether it would work or not. Well, it works beautifully.

The past few days, I've shoehorned a LiPo charger module into the device and fit a 500mAh lipo cell into the battery well. With simple testing, I got over 4 hours of continuous playback from headphones. This isn't the best, but I still think this is better than AAAs, which had a battery life of 5 hours for non-rechargeable alkalines and 2.5 hours for rechargeable NiMH cells. So I'm planning on redesigning StereoBoy R2's PSU to use LiPo batteries instead of AAAs.

This means future StereoBoy models will only be able to use aftermarket shells with modified battery compartments, not the original shell. But aftermarket shells are available easily for cheap online, so I don't think this will be a huge deal.