I don't know how much weight you want to add to the gun. But adding 16ga inserts won't add much while complicating shell availability quite a bit. Get some of the lead stick-on tape the golfers use to add weight to the back of clubs and put it under your forend. Should be able to add quite a bit of weight cheap and easy. Further you could position it along the underside of the barrel to determine where you need it for your tastes. Before you put it on be sure you know what solvent takes it off.