There was a recent thread on nuc construction. Sumary of original.
Mine (only 3 in total mind) are mesh floor. Done on the basis that I can always close the floors on them in if I need to (ply board and gaffa), but I cant open them up. Varroa drop was prime consid, sound ventilation given nuc entrances can be quite small / closed for moving was the other. My hive stands have a side skirt so this acts as a bit of a wind break as well. Light top ventilation vis perspex crown boards with vent holes and roof spacing. Crude entrance adjustment desirable as well (open, half, closed). R