A padding I've been using is the flat sheets you get where they sell swim noodles. They are light, 97 gms for a piece 3/4" x 5 3/4" x 58" long. About 1.5 lbs per Cu ft.

It is easy to cut with an Xacto knife and it is fuel safe and no-porous. I recently had a fuel tank rupture and dump four or five ozs in my fuselage. I pulled out the foam padding and dried it off with a paper towel. None was trapped in the foam. Can't say the same for the wood though.

Don