Making DIY exterior thermal window covers

[Green Giant]

We are thinking of making Two exterior thermal window covers, one for the sliding door and the other for the kitchen side.
I’ve found the material for the covers but not sure what is used inside the cover to add a thermal layer? Any ideas,
We have a front and rear tailgate cover from rainbow screens and they work well.
Ta

 

[Deaky]

This stuff is what I used for internal ones. It’s waterproof so would work outside

https://www.screwfix.com/p/ybs-airtec-reflective-double-insulation-25m-x-1-2m/6520P?tc=DA4&gad_source=1&gbraid=0AAAAAD8IdPzHK7e4ztNmybZKWgwsN2llm&gclid=CjwKCAiAg8S7BhATEiwAO2-R6nNuJtkVCCUPSh_nKbXI8zR5QSN3d7TrBkYaSwAP7aalJ2gqV6SyoRoCERAQAvD_BwE&gclsrc=aw.ds