iPod classic 160gb modded to 256gb with 2000mah battery and Bluetooth. Just waiting on a 3d printed back. All fits within a thin shell. Very happy with how it's come up. :
1x 3000mAh Battery Replacement iPod Classic 6 6th 7 Video 5 5.5 Thin U – ResaleFirm
iPod Classic Back Cover Replacement Kits & Services
Upgrading my iPod Video with a T H I C C 1950mAh battery (and other stuff) - YouTube
3800mah Battery Upgrade Replacement For iPod Classic 6 6.5 7 Video 5 5.5 Thin* | eBay