Hope this helps. Cushions removed. Heres the seat being lifted (refer back to previous pages for steel construction)
Heres a closeup of how the seat is locked in position so it can't come adrift and send rear passengers into unexpected 'auto-bed' mode.
In fact its a snug fit and the weight of seat base and passengers actually stops it lifting out although a simple bolt catch could be added - I just saw no reason to fit one as it works perfectly the way it is. When in seat position, theres about a 4" gap between base and back to be able to pull through lap belts quite easily. When its down, they disappear underneath

Likewise if you're wanting to sit in the back for a meal, the belts are easily pushed through out the way. Seat base and back is only 11mm ply to keep weight down.
(Below) And with bed down - perfectly level. I have slept in it and its very comfy. I'm a big lad and it sleeps me and one other fine. Support for the leading edge is partly on a strip of wood along a cupboard (unseen in photo) over on the left although I may hinge fascia of seat to seat base to make a full-length support - not decided yet. Back of seat is strengthened with a piece of profiled steel fixed to rock n roll brackets which as I say, drops nicely into seat base when bed is down if you leave enough space at ends. Beats an MDF one by miles and now I'm perfectly happy for kids to sit on back although 3-point belts would be better still. Main thing is that seat is going absolutely nowhere now.
Lots of ways you can do a rear seat, mine being only one suggestion but I was looking for a level bed with good strength, excellent anchorage points, requiring no support pieces in centre of seat (ie clear underneath) and one that won't fall apart or get ripped out easily. Much of it came about during the build stage, spotting minor modifications to get it right. It also forms a strong cage around the tank and (arguably) strengthens the back of the vehicle in terms of a side impact (MDF would offer none at all) - just my take and each to their own on this. However, the construction is all here for anyone who wants to follow the idea, plus I think I put it in the WIKI a while ago (may need to update that). Materials were mostly donated/borrowed/found. I guess if I'd bought all the materials new it might just come to £80. Another bizarre Covkid project over and done with.
In the same week, Chickenkoop & Big Gordon (my neighbour) sorted out a tow bar for me this week too - out of odds and ends. Bumper was removed, a length of chunky 3mm box section welded between existing bumper irons (ie right behind bumper) and a shorter horizontal piece to weld tow hitch to. Works a treat and very strong!
Apologies for the crap round of coffee I made lads.
The tow ball and plate was donated by a family member. Welding carried out by a pro (Big Gordon) using industrial welding equipment rather than a lightweight affair to ensure strength - you don't take ANY chances with a tow bar. Electrics to follow - trailer socket and additional footlights in seat base.
Incidentally, I have a wholly 12v system which gives me more flexibility if theres no hookup on a site and even where there is, I have a portable 12v 17amp transformer which keeps up with anything I'm running. All lighting is low-consumption LED stuff and only ever buy 12v appliances.