Ditto: my long hive has mostly top bars, but takes 14x12 frames. The 14x12 Tanzanian shape is a little on the large side for handling unframed comb, and they do like to fix it to the sides (easily separated, so not a huge problem). In fact, I may use some more foundationless "empty" 14x12 frames next year, as they're easier to handle than a great big rectangular slab of unframed comb, and I occasionally like a close look at the brood. They had a couple of empties this season and filled them very quickly; however, this does rather lessen the convenience and economy of the bars-only TBH. For that you really need the sloping sides, I think. Also the graceful loops of comb are very aesthetically pleasing!
I don't have it now, but when I made a TBH with sloping sides (i.e. the Kenyan sort), I was foxed by the many suggested angles of slope and ended up draping a dog lead from one side of the top runners to the other, and fixing the sides to match the angle of drape, as it were. Which is, I assume, a fair approximation of a catenary curve.
My home-made long hive has done really well this year, and the bees are so easy to handle. Extending their space is a doddle, and you can look at sections of the hive without exposing the whole nest, which is the big snag with Nationals as far as I'm concerned. I shall be building another long hive for next season, and probably won't get any more pricey little boxes in future. But each to their own: a 4ft hive is tough to move about and perhaps wouldn't suit most commercial beeks.
