I've flirted with the idea of building one of these, but it finally took a client to give me the final shove. He has a Regency TR-1G--the one that he'd had as a kid--and wanted me to see if there was a better way to power it. Eveready still makes the 412, but they're short-lived.
My replica takes up the same space as an original, but uses two 12-volt garage door opener A23 batteries, easily replaceable. I even made the outer sheath look like Regency's own version, the Number 215.
Despite the higher voltage, my TR-1 took in the battery with open arms, and played for the first time in ten years.
Pix and video links:
http://www.youtube.com/watch?v=5KnxS...e=channel_page
http://www.youtube.com/watch?v=X_ouo...e=channel_page
It's just about the smallest replica battery I've ever done.
Bill