Amazon now carries the three AA to D battery adapters that look the same as the ones carried by Battery Junction but list them at 5 for $11.03 rather than $14.95 each.
Parallel Connection:
Serial Connection, cheaper price, five for $5.54:
I have used identical looking yellow three parallel AA holders I got from a ebay source in my Fenix TK50 light and got 3 hours run time on maximum output with them. As they are set up parallel they work with one, two or three AA batteries installed.
Excellent find. Bought some of the serial guys. Makes gifting a MT-G2 2D Mag to a non-flashaholic a possibility. Thank you :)
They also have some 4 AAA in parallel battery holders which appear from the listing to be about the same size as a C size battery that some people might find useful.
http://www.amazon.com/Parallel-Connection-Battery-Plastic-Holder/dp/B00K86S012/ref=cm_cd_al_qh_dp_t
Per the listed dimensions these may be a tad short to perfectly replace C batteries but I can make spacers if necessary.
Grrr. There were only 4 left when I saw the thread. I opened it in smile.amazon.com and there were only 2 left. I hurried to find a couple more items to get the $35 for free shipping, ordered. Checked the product page again and now they're in stock for $10.33 with free shipping no minimum. 
Thanks for the find, though. :beer: