If your space problem is the length of a 18650 battery, maybe a 18500 would work. I am using two protected Keeppower 18500, 1700mAh, (in series) for a segmented string blade and I am satisfied with them. I think you can find 2000mAh too, and even if it's not the same as 18650, for me it was the only option to fit inside the hilt.