I think for one thing, you want more that foil to cover the pan. I had covers made for my bigger pans, my flues pan has a hood, the 3x3 syrup pan and my 2x6 finisher I had a tin shop make covers, the bottler came with a cover and I made covers for the 6 boxes, float or draw of between 3 pans. This includes any float boxes or draw off boxes.
I can't help you with leaving partially boiled sap in the pan. Just a guess, but if you draw off, once it cools enough, what is in the last channel and pour it into the first channel, it will be protected from freezing down to?. That temperature will depend on how much sugar is in the pan.
While I'm at my sugarhouse daily in-season, if I get an extended freeze, I have never drained the pan or pans, as long as I boiled for at least 3-4 hrs. At times slush will form on top, but never solid ice. In my early years I actually placed a 60 watt light bulb in the firebox, but after a couple of years I found that not necessary. Some of this will be related to the outside temperatures, if maybe 10F or higher, no issue, if -20F ?
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.