I spent quite a bit of time trying to work out what would fit - and work. In the end I made a fairly impulse buy and bought (£170) a rock/roll type bed from Atlantic Fabrications
. I think they only sell via ebay, but I went to the factory to collect.
A couple of decisions. I did want an MOT-compliant type. In the end, the much lower cost, and some uncertainty about whether it would be a good fit, I bought this 'basic' version. It means that I can't carry passengers on it, but there is room for the third seat should I need it. The second decision was size. There really isn't very much room in the van, and I wanted some space when it's all folded out. So I went for this slimline version. And my partner's got a T4 anyway, so we use her van when we go away together.
The basic frame is pictured below. Very nicely welded together and sturdy, and I assembled it in about 20 minutes. Disassembly is straightforward - undo the four hinge bolts and it separates into 3 sections. The frame is built of 15mm and 20mm box section steel and a 6mm mounting bar. The coating is paint - not powder - so likely to scratch. It weighs about 30kg fully assembled, adding about 10kg overall. I reused (12mm, 9mm would have been fine) ply and cushions from the old build. The unfolding mechanism involves lifting the seat front and sliding the lower frame out. It then opens flat, located with the hinges and guides within the frame. Folding up - lift at the point between the two hinges. Flat as a bed it's about 184cm x 75cm (just fits, a few cm to spare, with both seats forward). Up as a seat 110cm x 75cm.