Great examples, Hacker.
Sometimes the cheapest and simplest solutions are the best.
If anyone has difficulty obtaining this type of foam sheet, it is sold as laminate flooring underlay by many suppliers including Howdens. You can buy industrial quantities quite cheaply and share amongst your friends.
(Beware though because some of it is green rather than white.)
If anyone within reach of Chateau Silkstone would like some, I have a roll of it left over from the recent refurbishment.