I have used original Zest frames and also tried 12 x 14 frames. In the Zest hive the bees are out and about much earlier than in a traditional National hive (I run both). I often used the hive as a source of extra bees when making up Nucs. Original frames are available from Bill Summers at...
I feel that the best option is to place the new OSB below the single overwintered box, preferably using drawn foundation. My reasoning is that if placed above, the bees may tend to move stores up into the new top OSB wasting time and energy. As I have several colonies in single OSBs I may try...