If you build them that the outer dimension is under 7 3/8, then two will fit side by side on a bottom board inside the rim so they are sealed flush.
That leaves a nominal inner cavity of 5 3/4 which easily accomodates 4 frames. I build them out of dirt cheap incense cedar fence boards, which are 5/8 thickness, so I have a bit of extra space.
The ability to fit the nucs side by side on a bottom board saves a piece of hardware. The bottom boards are more stable than other style nuc bottoms, and the screen boards give some early season insight into the condition of the nuc without out opening it. Splitting a 10 frame with a insert of coroplast would do the same and let you squeeze in 5 frames. Just handy to have the nucs ability to restack.