Insulate the no-hole coverboard, so that it stays warmer than the walls - that will stop any condensation from forming on the coverboard and then dripping onto the cluster. Condensation occurs on the coldest surface.
The insulation layer needs to be pretty good to be effective (the bees don't generate much heat.). Modern building insulation board (Kingspan/Celotex/Recticel/etc) is massively better than bits of sponge rubber or old carpet! And insulation board is cheap, even if bought new (£3 per hive?)
Remove the inspection board. Allow any debris to fall clear and any excessive condensation to drip clear. (Condensation on side walls isn't all bad - it allows the bees to recycle their water rather than taking on the dangerous (bee-lethal) task of foraging for (cold) water in cold weather.)
If your hive is on an exposed site, with a gale of wind blowing under the hive, then consider putting an empty super under the floor, so that there is sheltered still air immediately below the OMF.
Insulated (closed) coverboard plus open (mesh) floor works well in English winter conditions.
//ADDED I'm sure it would do equally well in lowland Wales. However, in the more mountainous areas, a polyhive (likely still with open floor) should do nicely.