Have not seen a post on BYC about it or any prefab nest box, exc those in ready-built coops and those are factory made/built-ins. Most just build their own, or use different things such as 5 gal buckets on their sides, milk cartons, etc.
Size should not be under 12 x12 inside dimensions. I stumbled onto some surplus plastic parts bins that are 1 x 12 x 4" deep and they work fine. Easy to take out and hose out if needed. So far am really blessed in that my chooks never poop in their nests.