National into Langstroth does go, but not vice versa.
greengumbo on here has just bought a load of poly Langstroth boxes ... he chose
Swienty, which seem universally liked by those who use them.
And if you choose National - for convenience and compatibility and size - the Swienty boxes are compatible with cedar Nationals. This is more than can be said for a number of alternative poly hives including the Apimaye Thermo Beehives which - with 23 separate pieces in their exploded diagram - appears to have about 16 bits too many for convenience and sanity ...
Floor, brood box, queen excluder, super, crownboard, roof, frames = 7