Hey buddy,
I posted a pic of my pen, so you can have an idea of where might be useful to start. The materials cost about 100$ in all, but since it sounds like chucks are your main concern you may be able to save money on the wire screen. get 1/2" mesh or perhaps a bit larger, but remember chicken wire doesn't always last. It gets brittle after a while.
As Tom said, brick Chucks=Roasted lizard I collect medium (read: easily moveable) sized rocks from their native hillsides, (a sort of comfort thing, although I don't know if its actually comforting) and construct a firm pile. This allows for their hides. They will dig. make sure you set whatever enclosure you make firmly in the ground, even consider making a barrier underneath your pen to decrease the ability for jailbreaks. Take this from a guy who lost his entire collection from a Tortoise burrow collapse. do that, or construct a bottom to your pen.
I have toyed with the idea of fashioning a larger pen, because I'm not sure even 12'x6' is really big enough for these guys. Perhaps making it taller would be the key, but I don't know. Just remember the more space/hides/natural heat you can give them will dramatically increase or decrease your success as a Chuckwalla keeper. Also, as Alice said, consider planting food plants for easy non-pesticide snacks for your guys. Right now the spring ephemerals are in bloom and the chucks love them.
As Tom and Johne said, natural sunlight does wonders, I will attest to this as well. Helps their colors, attitudes (actually it keeps them mean =) ) and health. I would consider keeping them outdoors all year if I were you. They are wild and know how to take care of themselves. my thoughts anyway.
Chucks are demanding, bro. I have had mixed success with them, (since they all escaped) I will say this, at least they aren't Horned Lizards.....I have faith that you'll be all right with them if you take the advice, but seriously if the lizards don't eat, or you can't finish an outdoor pen this week, take them back. Go out and get them back when you finish. Its better to be prepared than flying blind. You can do it if you're willing to put out the effort, and we're always here. E-mail me if you need any other assistance that I may be able to provide.
Good luck man,
-Phil