The one that my girlie is in right now is a 10" pine cube. The front of mine is plexi, screwed in, so that when I need to remove the female (whether its at deposition or pipping), I can take the front off, and easily slide her out, eggs and all (as opposed to picking her up from the top.. you learn from experience :P). The front is covered with a cutout grocery paper bag for privacy. The lid sits inside, dowel pins keep it there. Also, a webcam affixed to the top. The hole is about 4" in diameter. All in all, it probably cost about 10 bucks to build. And this is a "proven" nestbox (first clutch hatched out of this one).

I put moss at the bottom. This time not so much cuz I ran out, and hopefully it has little impact.

I'm having humidity problems in the vision, so make sure your nest box ends up in an environment that you can control pretty well.

I think my next nest box that I build, I'm going to put something in for a water dish, something affixed to the side, attached with airline tubing from the outside, so that after deposition, I can see if the girl will go over there and drink. 

One thing I have noticed is that she is VERY mobile in there.. she'll be on the left side, right side, she's moving around significantly. I don't know if this is "normal", but it's awfully interesting. She will flatten or beehive high, depending on temperature, but at about day 25, she started to flatten out a bit, and has been coiled kinda strangely... it's weird having the ability to look in on her all the time, the behavior is neat, I don't know if it's typical or atypical. *shrug*

I've tried semi-opaque rubbermaid, with zero success. In fact, one girl laid her eggs on top, and coiled them there. She was easy to remove. :D

Good luck with your nest box :)