Howdy Rog
I linked you a flashing red LED from All Electronics in the other thread on this, but here is a bright red LED from the same source that's spec sheet suggests it will work between 2 and 4.5 volts so it ought to be perfect for this application.
BTW ... an empty .308 case works fine as a replacement for a AA battery. I've replaced the middle battery in my pack with one and am running right now!
Hope this helps!