Re: Horizontal Hive w/ Modified Frames
I make my own frame components, so if I were to do this I would just make longer end bars to make 16"+- frames.
But if you don't want to manufacture custom end bars from scratch, you could 'splice' two deep frames with the aid of a wood 'patch'. An easy way to do that is to buy individual frame components, such as these frame end bars:
Cut the bottom off one end bar, the top off a second end bar and splice them together by gluing a wood 'patch' over the joint. Put the patch so that it will be inside the frame when assembled. Use plenty of Titebond glue on all contact surfaces.
The easiest way to add mechanical fasteners would be staples from a pneumatic gun, but if you don't have one, brads or small wood screws could be used as an alternative. I'd predrill if you use screws.
The above is offered as advice on one option as to how to get suitable frames, not an endorsement of the size chosen. My suggestion would be to first try a horizontal hive with standard deep frames before you switch to custom oversized frames. There is a lot to be said for the ability to move resources back and forth between standard hives and the horizontal hive.
USDA Zone 7A Elevation 1400 ft