Thanks all!
I often forget how awesome Wikipedia is, and it turns out
this article gives the full breakdown on DC plugs/jacks. It indicates Type M adaptaplug = 2.1mm ID x 5.5mm OD. Seem right?
I realize I'm kinda wanked in the head for it, but I'm going with a SLA battery that weighs 6 lb.! It is 9Ah, and will integrate into my Pelican box along with a charger and the R-44 wall wart. Must have power!
Peace,
Sanaka