There is no way to add flotation to the back of the boat inside the hull. That is a function of the width, depth, length and shape of the hull itself. The hull is going to sink into the water until it displaces an amount of water that is equal to the weight of the boat. If the weight is at the back of the boat, the boat tips backward and the displacement happens all at the back, as you observe.

You can play around with the balance of the boat, but it sounds to me like you may have a bit too much motor weight for the hull. The only fix I can see is either fewer people or less motor weight. Sorry.
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