I make wooden nest boxes similar to the link SurburbanHomesteader posted, except I use the plywood on the bottom too ... I just wipe it clean with vinegar water and let sit in the sun for a couple of hours to sterilize.
I would rather a bit longer nest box than a shorter/wider one ... the doe will make the nest in the back of the box and when she jumps in, she doesn't stomp on the kits. The does also appreciate having a shelf to lounge on and I will give the does the nest box after the kits are weaned so they have the shelf. Without the nest material or kits, they generally don't soil it even if they use it to get out of the wind.