correct me somebody if i'm wrong but:
to run the series of 6 LEDs, you'll need 6*1.5 = 9 V, 300mA per series*15 = 4.5 A total
and the series of 3 LEDs is 3*1.5 = 4.5 V, 150 mA per series*6 = 900mA total
108 LEDs
total = 5.4 A of current required? that would mean a battery rated for 5.4 AmpHours would only last... an hour? if those calculations are correct then i think the bigger problem is finding a suitable battery (i'm not sure how portable it would be).
again, correct me if i'm wrong as this current seems rather large