Hiding duct work in new closet

My house plans have an optional closet in the master bedroom. This closet was not added during building but I can go in the attic and see where it would be. It is already framed, actually. I want to convert the space into a long closet but there is a problem. There are two flexible air ducts running through it.
The options that I can think of are: 1. Reroute the air ducts. This would require about 20 extra feet of duct so I am not so sure about this. But what if I converted the flexible duct to solid duct? Would that offset the extra length?
2. Push the airducts between the 2x4s in the wall and then build out an enclosure around the air ducts. This would result in approximately an 8" column sticking out from the wall. This seems like a good approach, but the closet will be narrow as it is so I am not crazy about it. And, is there any kind of restriction that prevents sticking air ducts between walls?
Any comments or suggestions are appreciated.
Thanks, UE
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Any time you play god with air ducts with out know EXACTLY what your doing your asking for trouble. 10 feet more may be enough for the air volume to suffer, maybe dramatically. Call a pro in your area, have him look at the situation.
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