You never told us the length of the LED strip needed (therefore the number of LEDs and the amount of battery current needed).
Well, my original intention was, to some degree, to adjust the length of LEDs in the costume based on the battery size it'd require - i.e. if I could get away with more length at a given physical battery size/cost then I would, otherwise I would shorten it. So I didn't really have an explicit strip length. All this confusion just started because the current measurements I was getting made no sense as compared to the numbers I expected based on calculation.
I still see all kinds of wild claims about how long this stuff will last too. i.e.:
***broken link removed** claims 4ft strips will run for 4-5hours off of a single 9V battery (4ft->0.2439 of a spool, 0.2439*21W=5.122W/9V=0.5691A. Since a standard 9V battery is around 550mah, I'd again expect it to run for less than an hour. But they say 4-5hrs.
***broken link removed** says their hood runs 12-36hrs off of a 9V. The fact that the hood changes color to me means it's 5050 (vs 3528 or something), and just looking at it the strip has to be at least a couple feet. How this could possibly go for up to 36hrs on a 9V when a 9V only has ~550mah is beyond me.
*Even during my own testing, my 5m spool has definitely run for *way* over an hour on this 8-pack of AA's, whereas the calculations my first post (which reflect every how-to-calculate reference I've seen - i.e.
https://www.reddit.com/r/DIY/comments/3abnek/trying_to_fix_my_electrofur_running_led_strips) show it should draw 1.75A and thus kill the batteries in barely over an hour.
At this point I've pretty much given up on trying to make sense of the numbers, and am just going to design with the shortest length possible, and test to see what happens. I expected this to be a super quick division problem, but unfortunately it turned into hours of research & confusion
In any case, I very much thank everyone on here for their help