To insulate I started with the sliding door. I am not sure how much difference it will make but it's cheap to do so why not. I started with a bit of flashing tape to help deaden panel vibration.
Then I cut Celotex to fit, I had to leave a few gaps as I didn't want the Celotex to interfere with the door mechanics.
Then I whacked in some thermo bubble wrap and sealed it with aluminium tape.