A classic winter DIY.
I made a double window using Corrugated plastic.
The window temperature is + 10 ℃!
It is recommended because it has an effect on condensation.

Material * Quantity varies depending on the size of the window frame.
Corrugated plastic 1820 × 910 × 4mm 2 sheets
Hikari Mall, on white glass door rail 5, 2 1820mm
https://amzn.to/2YJwAu8
Hikari Mall White glass door rail 5 bottom 1820mm 1
https://amzn.to/2Pztepm
Hikari Mall White Joiner 3 3 1820mm 4
https://amzn.to/2PxcVJE
Double-sided tape that can be removed
https://amzn.to/2P9VC2f