I looked at the Minn-Kota battery box, and decided to go with a plastic $10 box and a seperate circuit breaker. Cost about half the price of the Minn-Kota version. The Minn-Kota version does have easier access to the connection leads, you can unconnect the leads without opening the box.
The disadvantage to the plastic box is the method it holds the battery in place. You bolt the box to your boat, Drop your battery in, put the lid on the box and then wrap the strap around the box. The strap holds the lid on the box so teh battery can't come out, however the battery is still free to move about insdie the box. This may or may not be a problem, depends on your conections to teh battery. If your wires are thin or you have circuit breakers attached to teh post you could break them off if the battery bangs around too much within the box.
The only real advantage to the box is it keeps the acid in the box, and it prevents the battery posts from making contact with anything else. To improve the plastic box, use a bungy strap and two eye bolts inside the box to hold the battery in place.