So thought I'd share what I have done for our bed conversion for others who were looking at movable bed options. I worked out the conversion is about the size of a King Single (conversion is a bit shorter in length but same width). Being a bit on the hard size I bought a foam mattress topper from Target. I worked out the Queen size was perfect to cut in half and fit exactly. This way I have extra softness with two layers as it isn't super thick.
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So using a King Single Mattress Protector to hold the two foam toppers in place (queen topper cut in half) I now have a movable top for the conversion which can be removed and rolled up and the table put back in it's place during the day if we need to. It is slightly longer than the bed but I decided not to cut it to fit as I figure it will offer a bit of protection to the seat cushions and prevent dirty feet staining it.
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I bought king single sheets (one fitted and a flat) and am using the double bed doona that came with the van (as I put a queen on the double bed for extra width and less chance of being cold due to doona hogs
The topper also solved the issue of not being able to use a fitted sheet on the bed conversion cushions. Ours is very tight, I think they made the middle cushion a tad too big.