Lots of people make their own arks.
If you want one to last then try to avoid wooden frames. Eventually pigs will eat the wood. I'm not a joiner but attach with good fixings not ones that will easily pull out. Protect round wooden entrances with metal, again to stop chewing. Stand on railway sleepers to slow down the tin sheets rotting at floor level.
Have you got a seek and sell facebook page in your area? Can be a good place to find materials.