They need to think outside the box on this one.
Biggest problem is the mortar and a location to shoot it.
They need to find something that will work as the mortar and then design the shell to that size.
Makes about as much sense as building a bullet and then looking for a gun that can shoot the bullet.
Does it matter if the shell is 54", 55", 56", 57", 58", 59".
The mortar and location will be the expensive part.
If they could find a location out in the desert and install the mortar, they could reuse it each year.
I would check around with the scrap yards and oil industry for something that might be the right size for the mortar.
The oil industry use large diameter pressure and reactor vessels that could be converted to a mortar.
They do make large size HDPE pipe but it is made off-shore to order. They make fiberglass but it is very expensive for that diameter.
The mortar installation will need some major foundation work to absorb the lift charge.
A location with good solid footing will be expensive to dig the hole for the mortar.
A location with soft soil will be easy to dig the hole, but will require a great deal of foundation footings for the mortar.