My platform for sleeping on needed to be 175cm by 117cm and fold into three. It made sense that it be made of wood - light enough to fold and carry out but strong enough to carry two biscuit loving peoples.
Went for some 9mm plywood from and timber/DIY place - and got three 58.3 x 117cm pieces cut. To attach these I decided on piano hinges - used on opposite sides, so when folded it would make a 'Z'' shape - fitting into the boot.
Then to fit the platform - it folded out fine in the Berlingo but there was a gap between the front seats and the folded back seats (the foot well basically). This needed support and had originally decided on some kind of attachable legs to prop this up - but with a spare bit of wood lurking, decided to use the front doors as support*.
*Was worried that opening the front doors in the middle of the night would collapse the whole bed - but it stays in place until you prop it back up onto the door handles.
At the hinge point there was a slight bit of flexibility - so to make that a bit sturdier, I added a spare bit of wood and a duvet. Our feet would be down this end so it was fine for this purpose - the top 2/3 of our body was supported by the sturdy Ikea boxes.