Increasing the Range of my E bike

I just put a wilderness BL-36 kit on my Schwinn beach cruiser. It runs great, gives me a top speed of 23 mph. on level ground and I am able to buzz around town running errands and such without draining too much out of the battery. I recently got rid of the flimsy battery bag that comes with it and replaced it with a small plastic, sealed lockable storage box. I placed the controller in front of the box inside a 7 oz. sealed food storage container. I have everything as weather resistant as possible for now, but would like to upgrade my range. I am told the kit has a range of around 20 mi., but I suspect it is probably more like around 15. I haven't tested the limits of the kit yet, but I was wondering if anyone could tell me of the cheapest, most effective way of increasing the range on my bike from 20 to 60 miles. I would greatly appreciate any and all input.