Guess what? I measured one of my top bar hives. Eveything is made from rough cut 1 by 10s. Remember that bees just need a hollow space and dimensions are not important, though this size has proven versatile. To generally keep combs straight on the top bars, they are cut at 1.25” width. This is just the general trend, and NOTHING IS ABSOLUTE WITH BEES.

INTERNAL DIMENSIONS (approximate with rough cut lumber)
Width of top (accommodates a langstroth top bar): 18.25”
Width of bottom: 8.25”
Angle of sides: 120 degrees, gap left at bottom of board makes the side 10.5"
Height: 9”

Top bars: 1.25” by 20” (accommodates 4 popsicle sticks)

A .25” by 20” strip separates the divider boards from the first and last top bars and maintain bee space.
I sit the side boards angled onto the bottom to create bottom entrances. See some pictures. Get creative.

The box is 3 feet long, for convenience. 4 feet would give them even more room.

Born to bee bad,
sam