When making my first Warre hive I decided to use standard Langstroph frame components for my top bars and cut a section out of the middle of the top bar and then doweled the two sections back together to the correct length. I did leave shortened side bars, as Warre mentioned in his book, and this prevents any attachment to the side walls. The assembled top bar with side bars also allows for the correct spacing of 35mm. I also put in a mesh floor as in Auckland, New Zealand we have a very mild climate and need that ventilation.
From the photos attached you can see the underneath of the boxes