get 3 bits of wood slightly wider than the spread of the legs of your camp bed, and cut a slot for the feet to sit in, that way it spreads the load, you get the lie flat bed you want and no damage done to the floor of your tent
the sun loungers (in the first link) do lay flat, you have to fold the top part- (where your head would be) all the way forward , then fold it backwards until it clicks, then forwards just a little- then back again until it clicks, eventually they will lay flat, unfortunately when you sit on them , the head piece comes up again , so you have to hold the head part down until you get on and lay flat, yeh, not ideal but they do work
Thank you all for your responses.
Marg6 - Thats so simple, great idea.
G#Winger - My Father in law has got one so will have a play at the weekend.
TimCESmith - Well spotted.