Cushion sizes for ford transit duetto

Hi is anyone able to help we  have a ford transit duetto , we need to find a cushion so we can make up double bed,looking at sizes in instruction manual we think it's the number 5 cushion.   The sizes seem to be 540×320×115 
We would appreciate any help 
Regards 

Mm

My cushions are:  small one is 43cms x 27cms x 19/20cms - top.  
                                             43cms x 27cms x 19/20cms - base
I think this is the same size as the one that fits behind the drivers seat to make up the single bed.
  Large cushion  is 70cms x 36cms x 19/20cms - top
                           70cms x 27cms x 19/20cms - base
From memory the large cushion was in the middle of the three to make up the double bed, but is suspect is interchangeable.
NB.  The drivers seat must be as far forward as it will go in order to insert the cushion behind it.   It took me several months to realise I did not have it far enough forward to fit the cushion so that it was flat!  A liberal doe of WD40 on the runners solved my problem!   
These two cushions are stored at home as I travel solo.
Hope this helps.
Sue

There are three small cushions that make up the bed; one has a soft base, one has a hard base, and the third has a hard base but also has a cutout in one corner to accommodate part of the passenger seat. 

Which one are you missing?