There is really only one rule you need to follow when making a top bar hive and that is to obey bee space. Bee space is any space greater than 1/4" and less than 3/8" in the hive. In addition to this the bees build thinner combs for brood and thicker combs for honey storage. Other than that, my thoughts are to keep the design simple and easy to make.
(2) 1"x12"x8' (sides, ends and follower boards)
(1) 1"x8"x8' (bottom board)
(8) 1"x3"x8' (top bars, top bar guide strips, spacers, rails - make extra top bars)
(2) 2"x3"x8' (legs)
(8) 5/16" x 3" hex bolts (legs)
(16) 5/16" flat washers (legs)
(1) bottle of wood glue (food safe)
(1) box of screws for everything else
**Something for the roof (not included in parts list)
Step 1: Make top bars. The top bar is 17" long and between 1-1/4" for brood comb or 1-1/2" wide for honey comb. You can also make all of them 1-3/8" wide if you want a more uniform bar. You can also cut your guide strips that will go down the center of the bars - I like the chamfer molding style for a guide.
Step 2: Make the follow board. Actually make two so we can use them as guides for putting the rest of the hive together. Attach the board to a top bar so it's uniform with the bars. The board is about 14.5" at the top and and 5.5" at the bottom and 10" tall.
I like to make these with plywood because solid wood tends to warp in the hive.
Step 4: Put the body together. Flip the two follower boards upside down and rest the sides on them so that they are snug to the follower board and on top of the top bar. Make sure everything is square and screw both ends to the sides.
Notice that the sides are resting on the top bar of the follower board.
Add the ends and screw them to the sides.
Step 5: Add the bottom board. Screw onto the ends and the sides.
Step 6: Add the rails. These are strips of wood to protect the ends of the top bars that hang over the sides. This helps prevent the top bars from being damaged when putting the roof on.
Step 9: Make something for a roof to keep the top bars dry. My general rule is that nothing should set directly on the top bars and they should be able to breath, but also keeps them protected from the elements.
You are ready for bees!