I agree with Piggiemom.
Sadly, before I learned about CC cages, my pigs would chew on their wooden house because they were bored and depressed. So your cage plays a big role in their happiness.
Also, the bent grid houses are very easy to make. In the hot tip section, there is a person who says that you can put book covers over the grid, which does work. It's very cheap to do and I assume it works great.
You can also give them a cardboard house (Like Piggiemom said) and cut two or three entrances into it. It's a big hit with them. =)