Had a bit of a play this afternoon with some scrap 19mm ply I had laying around.
1hr later I had a 4 frame flow hive that will fit straight on a 5 frame nuc
One thing I will change if I do another is the bottom metal brace, used 19x3, frames need to be hard against the harvest end to overlap the plate and make it bee proof. Will go 25x3 next time.
Still need to close the small gap either side of the flow frames to make it bee proof, a few paddle pop sticks should be just the right thickness.
Using 19mm instead of the 21/22mm thickness of most commercially made hives means if you stick with the same outside dimensions it is the perfect size for 4 frames. If you stick with the standard 5 frame nuc internal size of 200mm they are a few mm to small to fit 4 frames.