I am very fortunate that my local hobby shop in the UK stocks 3 & 6 mm Depron full size sheets.
Depron is Expanded Polystyrene (XPS) but does not have a paper backing however the way it is produced does give it a slightly denser smoother "skin". This makes it quite a bit more rigid than a ceiling tile or sheet sliced from block XPS.
The original Depron, it is no longer produced, had a rather denser skin making it ever more rigid.
The rigidity of Depron, even the modern type, means with the right techniques that it can be used structurally.
A structure with a 3mm Depron skin can be strong enough with just a Depron internal structure without needing any further reinforcement.
This Bombardier Dash 8 fuselage is basically a hollow tube of grey 3mm Depron with close spaced 3mm Depron frames.
The complete painted fuselage with wing mount, fin, tail plane and servos is 6" diameter and 77" long but only weighs 12 oz
Hard to imagine a lighter way of making it.
Just for interest there is nothing
in the fuselage. The motors, ESCs and LiPos are in the nacelles. The receiver is in the wing.