If you go with a rear conversion (you will lose the rearmost seats) AND if you have separate middle seats you would be able to go down the hammock route or single beds (based on flat seats).
Most rear conversions have the head end at the rear with your toes being the front seats.
We sleep with our heads behind the front seats
In some vans the front seats will lie flat giving you another bed options.
Some users use hospital type wedge shaped cushions to 'level' the seating area for sleeping.
I have not seen single rock and roll beds, but they may exist.
Rock and roll beds tend to be rather slim, although there are wider versions available.
Hammock wise, I use an American extra wide (1.5m) heavy duty hammock for our dog while travelling.
It is slung between the front and middle seats.
We don't use it for sleeping, but could as my wife is a shortie.
Similarly, I have seen 'adaptations' that hang from the headrests and/or ingress and egress handles.
The hammock approach does not use the attachment points to carry your weight, that is caarried by the seats and mini mattresses.
Your requirements are a little unusual, as were mine, so think carefully before making your choice.
Good Luck