I was browsing the internet and found a company that makes western red cedar brood boxes and they are only $17.50 a piece! I ordered 4 of them. You do have to pay shipping which ended up being $30 for the 4.
The name of the company is Naked Bee Hives. I just received them today. The wood quality is similar to the flowhive. The match is perfect as it is western red cedar. The size is perfect which I was worried about as I thought the flow hives were slightly wider than a standard 8 frame. Apparently not. I put together 2 of the boxes. No extra rasping or filing required. As far as QC, one of the boards out of 4 boxes was not predrilled, must have been missed.
All in all they are pretty much exactly the same as the Beethinking/flow brood box. It ends up being less than half the price after shipping.
So if you need a second or third brood box for your flow hive there is another option. As a disclaimer I am in no way affiliated with this company, just passing on a tip.
The brood box is the bottom with the flow hive super on top.