You could probably have one made from malleable iron (a heat treated type of cast iron) which is substantially less brittle than ordinary gray iron. I would think that would be more than sufficiently strong. Another feature that would drive up the price. This is definitely a field where if you have to pay someone else to do the work, it is going to cost you.
Also consider that a 13" mortar weighs 17,000 lbs, both from a materials cost perspective and a moving perspective.