I just have questions so I know what shop to point you towards because of production methods,expectations (longevity, material/durability,how omportant you forsee the ability to repair or would client opt to replace,etc etc)
You could find a prop manufacturer or marine construction company (someone with a cnc foam cutting machine) cut 2 matching halves (2.5d) & secure the halves & have it hardcoated.
You could locate a sign company who has a gantry deep enough on their cnc or the technical know how to laminate multiple sheets of hdu,etc..many sign shops with cnc routers do not have software that can do true dimensional work (many software programs or cnc operators/designers use their machines as what I would explain as 'fancy jigsaw' machines. But this would be an expensive option, gluing up multiple machined hdu panels (lots of machine time,lots of labor in finishing but with the right shop would yield a showroom artwork piece.
there are cnc spindles & 4 & 5 axis machines,it is going to depend on the finished size I know you stated the length 60" but what is the circumference this is going to determine who could fabricate in that method.
what I have seen of your work it is high end,how 'perfect' does this need to be?
alternatively if you think you will never have two of these parked side by side or there is a little tolerance allowed you could have a craftsman like signmeup create a one off,he does great dimensional work without a cnc machine.
is foam durable enough? there is obviously HUGE differences in materials & prices & considerations between HDU & say EPS with a hard-coat & would either be acceptable to the client,or does a master need to be carved & fiberglass parts created?
those are what I am trying to understand so I can suggest shops & fabricators who can provide a product that meets your requirements & customer expectations so you can contact them for quotes.