I had a full length expedition rack on mine for 3 months didnt notice any difference with fuel consumption but it must make a difference due to weight and drag. The reason I removed it when not using it was due to the really LOUD whistling wind noise! and problems with height restrictions at car parks etc.
Marine ply is also known as WBP (water/Boil Proof) Ply and is about £25 for an 8'x4' sheet of 18mm Unless you're planning on having a party on there I would have thought 12mm would be enough as long as the bars are at 400mm centres or there abouts.
I stand corrected,and glad to be so!Marine ply 18 mm for £25!Last time I wet for some in bristol (some years ago) it was closer to 100,I kid you not
I was in the sound industry building large loudspeaker enclosures and regularly used hardwood or preferably birch but marine was useful for outdoor stuff or high humidity enviroments....Im very glad to hear its come down.I'll be able to use 9mm marine to replace the floor rotting sponges in the back on my discos now,instead of tanalised.
Nice one,Red