Personally just put a sheet of insulation on top of the crown board (having closed the feeding holes) and slotted in a mouse guard. Have left the area inside the lifts empty as folk here have emphasised the need to keep the top of the hive warmer than the sides so that any condensation starts on them.
If a strong colony on a brood and a super, do as as Heather says.
As boxes are not as robust as in other formats, or if the colony is not so strong, a sheet or large towel or old coat would be good to drape over the brood and food (as well as that top insulation). If on a brood and a super, I would not reduce the OMF except possibly for short periods of extreme cold, but if just on ten frames, some reduction may be OK, or even a good thing.
Insulation does no harm. It can be removed early in springtime if you want. I never had any bothers overwintering even very weak colonies in my WBCs.
With my WBC's I just pop some insulation on the crown board. This can be sheets of corrugated cardboard or anything really; a little weight be it bricks or an empty super will keep it together. The lifts and roof keep everything dry and cosy.