I am sorry, I do not have a picture of what I did here at the office, but here is one that was posted on a BBS that is very similar to what I did. Except, I made my "frame from 2" by 4"'s and they did a better job. Done again, I would do the frame from metal like shown. After 2 winters, the wood started to mildew. Not sure who took the picture, or whose pond it is.
I do not have that pond or BS running currently, but just the temp pond with plastic crate BS going. This year, so far
, I just got some 1" thick insulation sheets from Home Depot and cut them to fit tight against the sides of the BS. I ran Duct tape around the structure to hold the sheet against the side. Works great, looks like hell, and lets a little air in. I will take and post a picture if you make me.
It was easy, we are moving in 3 months, and I am lazy. That added up to this way. Longer term, would do the polycarbonate sheets, and metal framework. In fact, in the big pond next house, I will have that on there all year round, since the BS will be in an out building and I want to minimize humidity damage to the building. previously have seen that rot the metal in an outbuilding . Plus it seems enough air gets in to do most, if not all the degassing. To me anyway.