After talking to some people (and after getting some very very beautyfull dark-oxblood-brown leather) i changed my plans a little bit. The first Tina i will make will be a more traditional hexagonal design (depending on what reeds i can get, first a miniature wih 8 or 10 buttons to practise the building process and then a 30 button).
And the square Design? Postponed is not abandoned.
What people were you talking to?
Hexagon was an apporximation of cylinder, in believe that radial placement of reeds will result in even tone. It was a mistake. If you are planning to use accordion reeds in the reedbanks, hexagon will result in unnecessarily large size, unused space inside, or crazy arrangement of reeds with resulting uneven lenghth of levers. Then you'll need uneven springs to make all action even. Why bother?
A square design of yours looks very unique, pretty, convenient. Instead of building it with 10 buttons, why not make 1/half row instrument, with 16 buttons.
5 in each side for main row, and 3 on the second half row? You'll end up with fully chromatic instrument, suitable for any type of music. Make it pitched in G, for better sonority. The only reason they build it in Hexagon shape is to make it look like antique. Customers want it. It's like making modern automobile looking like a steam engine. If most buyers would like it, they'll be made like this.
If you are building it for yourself, why listen to market oriented?
And if you want antuque looking, why not just buy one?