I had an 8 frame box with 9 frames. Me being economical in work ethics (read lazy) I achieved this by taking about 10mm of each of the two outside frames. Can’t remember exactly how much I took off I just sanded them with a linisher until they fitted.
Problems: while initially the frames were easy to move in and out once they were exposed to bees it was another matter. The frames I believe absorbed moisture , as they do, and it all became very very tight. And that they were full of bees and foundation when it became evident, made further work on the frames impossible.
I removed the two outside frames, salvaged some honey and reverted to the 8 frames with a follower board.
I may have persevered with this except I aim to have all my frames X wise and not length wise.Edit: See link here 12 Frames Crossway in an 8 Frame Lang Box