You have a few design options:
1. Make your bulkhead moveable on a rail system and attach hinged drop down bunk(s) to the cab side. Good but not useable when fully loaded, fairly involved to fabricate.
2. Keep your existing bulkhead and cut a narrow slot out of the top and make a platform into the load space. You sleep partly in the cab, partly in the load space but not much headroom. Easy to do, not much headroom but lots of legroom.
3. Keep your existing bulkhead and make the top third or so hinge ninety degrees backwards into the load space with a collapsible fabric hood a bit like a pram's to help keep the heat in. You would then sleep on the platform. Easy to do, legroom possibly an issue?
4. Abandon ideas involving modification of the bulkhead and have a longer bed with on cantilever folding arms in the load space that stores against the roof. Good for legroom but means sleeping in the back, you might not like to do that for various reasons.
5. I my T1Ncan (which had a bulkhead like yours does) I cut a sheet of ply to fit into the front seat squabs. It made a kind of floating platform on which to sleep in the cab. I stored it in a piece of U channel in the back so it didn't rob load space. There was a bit of a knack to bedding down but I found it to be simple and cheap and it barely detracted from the carrying capacity at all.
Interpretations of all these ideas exist in Europe. In my NCV3 I went a different way and permanently installed a bulkhead further back that has a four way bench seat built in, so it's either extra seats or a bed depending on what's needed (see pic). Occasionally I don't have enough load space but I bought a trailer to address that. There's always a time when you can't fit everything in, however cab space is regularly useful to me.
I tried the chunky profile inflatable mattresses and found them pretty poor. You can get some decent low profile roll up ones though, Thermarest are good and have a lifetime guarantee - I've used it three times in the last ten years or so! Packs down nice and small when not in use too.
Your main criteria will be determining what compromise you are willing to make in terms of load space versus cab space. Once you know that it will more or less dictate how you can proceed.
Here's a link which might give you some
inspiration!