I looked into building wings for my H-18. The bigger part of the fabrication job was the fittings. I just found a used pair via the internet but haven't recieved them yet. I remember that the main tube is 1 1/2" OD. The tube thickness depends on the design (bracing). Best bet is if someone has a set of NACRA wings that you can measure and get a pattern off of. A local metal fabricating shop will be able to bend the tubing without kinks and weld it if needed.
Put some feelers out on the internet for a used set and wait a while, you may get lucky.