We had a great experience with The Original Mattress Factory for our short queen innerspring. Only 50 bucks extra for the custom size.
No extra charge for the "hinge", which is not detectable when you are sleeping on it, but allows you to fold the mattress top to bottom for ease of entry to the MH, and is also a help when you flip the mattress.
The hinge was a no-charge extra on the queen, but the salesman said they routinely include it on king.