Found a new stuff for wing bags !
This is the stuff used to insulate wall beind radiators. Until now, the one I found was not very good for bags, but, there is a new range of tha thing in Homebase, called "EcoHome".
Comes in 4M rolls (60 cm wide) and cost 8 euros.
Looks like the bubbly plastic sheet you put under the floors, with a strong sheet of aluminium glued to it.
Can't tear it, this is very strong, and really thin (about 3mm all in).
No photos yet, but that is a good alternative to what's on top of this thread.