For actual sound deadening we've used Dodo mat hex sheets. This is A4 sheets of what looks like shiny silver bubble wrap that sticks onto the panels of the van and deadens the sound. Magic.
We also started to install horizontal wood batons (38 x 25mm treated timber) for the ply lining to be screwed to.
We have also put flashing tape over the wheel arches to further deaden the road noise