reduce the entrance to 10mm/1cm even a 2cm hole is too big and create a bad draft inside your hive, also reduce down to one box with honey on the outsides and brood in the middle, try your best not to open the hive as you just let valuable heat out, i use toilet paper believe it or not to reduce the entrance and also i use toilet paper and i stuff it in the ventilation holes at the top of my box as hot air rises and this is typically where it escapes, the good thing about using toilet paper is that the bees can remove it themselves when it starts to warm up, but each time you go to check on your hives i usually take toilet paper with me just in case ive had a lot of rain for example and need to replace a bit, but apart from that, thats all, dont keep your hives bigger than one beacuse with bees dwindling they wont be able to manange a double or triple hive!! regardless of how big they were in spring, simply there is just not enough pollen :P