Mike a, do you have a plan of your National sized top bar hive? Looks interesting.
My version is not National size as I decided to make it longer to give the colony more space and bars. I kind of made it up from memory as I built it and of course the lack of planning means I made several mistakes.
1.
Internal Depth - The space beneath the bars was far to deep almost 2ft. This would of caused a major problem when trying to lift out a single bar as the natural comb would of been extremely fragile plus I would of needed a step ladder to lift the bar out completely. Most common hives are no deeper than 12" for a reason, to allow them to be easily manipulated and to keep the weight of each frame down especially if it is filled with stores plus they have support all the way round the comb. The next one will be no more than 14" deep.
2.
Door - The doors had several design flaws, first of all I made mine from several planks of pine jointed together with biscuits and glue this wouldn't of been a problem normally but as it was only fixed by the hinges it warped outwards on both sides even though I treated it with linseed oil and wax. I should of made the doors out of one sheet of wood and the top edge should of been better protected from the rain and secured by more than one latch.
3.
Bee Space - Even if I had built mine to National size I didn't consider using a super on it at the time, if you look at the picture I posted earlier in this thread I left a small amount of space above the bars and as a National is bottom spaced this would of left more than one space when a national super was added so the colony would of probably filled this gap with brace comb and probably honey which would of made one hell of a mess when the super needed to be removed.
Another problem I can foresee is if the colony attach a comb to the doors, if they do there is no way of knowing before you try and open the doors if they have build combs on every bar. The only way round this I can think of right now would be to use top bars with side bars matching the slope of the walls or build a straight sided version and use normal frames which kind of defeats the idea of this hive.
I'm sure I would of found more problems if I tried to use it, but as it sat out in my back garden for a year and I never got round to modifying it so I recycling it.
The idea come from another bee keeper so I can not claim any credit for its design. Here is a picture of the original hive which is National sized which I based my version on.
So when I collected a suitable swarm I ended up going with another design.
I've posted a few videos in the video section if you want see more.