From your test data:
"-External, 8xD alkaline, wired in series for 12v output, with 0-15v panel volt meter; deck ran until voltage dropped to ~5.75v, though I didn't start with a fresh pack. I'd estimate ~15 hours run-time with new batts**, but the shutdown voltage reading was the key piece of information in this test.
[Edit: **After some real-world field use, I have some updated results. I was able to record 32+ hours on a new set of Duracell D batteries, and the battery level meter still showed about 60% (~10 volts). Yeah. Figure in another 20-25% additional time the unit was in standby mode for sound checks, and the capacity of this simple and inexpensive pack is phenomenal. I could use these same batteries next year and they could record another whole festival. No expensive RC batts and chargers for me, thanks.]"
This illustrates the advantages of using regular D alkaline cells. Namely light weight extremely powerful external powering of recording gear. I do have an equivalent very ruggedized 8 D cell battery sled made from no longer (available from stock) high reliability plastic materials.
Sonic Studios BD-8 model(s) can be fitted with appropriate power plug fitting the FR2-LE and other decks accepting 12 volts and working to ~5.75 volts end-of-batterypack-service voltage. BD-8 has option of disconnect from output cord connector so a different output power plug patch cord fitting other deck models is possible, and this makes the BGD-8, which is costly-to-make, a better investment as it typically lasts over 10 years of regular use without reliability issues.