Using Apivar mid-winter is going to be pretty hive dependent as it requires active bees to traipse over it's surface. So cold, wooden hives with no insulation are more likely to have clustered bees...so unless they cluster on the strips....
Whereas, bees in poly hives only cluster at the very coldest temperatures and are more active and move around a lot....Particularly at the top of the hive, the warmest place. So it may be worth putting the strips on top of of the frames rather than inserting them down and between.