A strip of aluminum duct tape across the oil cooler is all you'll need. CHTs don't vary much between summer and winter but oil temps do. I have factory plates and used them for about 30 minutes before I pulled them off and tossed them on a shelf.
Mine do, pretty significantly, about 50 to 75dF depending on the cylinder. But all I've ever done is cover the oil cooler with the FAA approved high speed duct tape too. When it's below 30dF or so, I cover the entire thing.
Try it at -30 or -40 some time! I have a strip of tape on my cooler for most of the summer. Two in winter. Do it for awhile and you'll be able to predict your oil temps by outside temps and tape coverage. Simple stuff. As for CHTs? I've never flown cold enough temps to make my CHTs cool enough to warrant any concern.
To the OP, make sure your carb is set up
rich enough for cold weather. Cold air makes your normal mixture leaner than in warmer air. It also makes more power and more power makes more heat. Some guys use air intake restrictors in cold weather to manage their mixture. I prefer to have my carb setup to handle it.
BTW, I've used regular duct tape for many years and the only disadvatage compared to aluminum tape is that it has more adhesive and that can leave a little goo on the cooler. Not a big deal but that's why many guys recommend aluminum tape. I usually use what I have in the tool bag.
Notice my cylinder has a baffle on it and the one in James' photo doesn't. My left side baffle is like the one in his pic. The right side has that shroud baffle. I've enjoyed very even CHTs since adding them. You should also insulate your induction balance tube and breather line. It helps for cold weather ops and doesn't need to be removed for warm weather.
Thanks everyone for the information. Was on the right track with aluminum tape and carb set-up; but always open to ideas. Never know what else could be out there. Just part of the winterizing. Not as bad as Stewartb, but the UP and Northern Wisconsin can be challenging. View attachment 48655