Mine fog if the closed-cell foam around them has worn, especially in
the bathroom where the frame comes out frequently to vent after a
shower. On other windows taping the frame to the window trim stops
fogging, but marks up the trim a little.
I made a similar frame with bird-proof acrylic sheet glazing to fit
around and seal the air conditioner. It has doors on either side of
the A/C so I don't have to align the frame, window and heavy A/C all
at once. I place the A/C in the window, unlatch the doors and install
the frame, close one door and push the A/C against it, then close the
other one. One side could have been part of the solid frame but two
identical doors aren't much more carpentry work than one.