Not really, but it is an idea. I"ll just take all that has been presented, modify it to meet my needs and then have a plan.
Was this the one?
https://www.thenewx.org/threads/budget-sleeping-platform.280411/
Your plan is the best one... take what you like from what's out there, discard what you don't like, and make your own solution that fits your needs. As you mentioned, cardboard is great for mocking things up.
I have spent some nights camping in the Xterra, sometimes solo and sometimes with my wife. We keep it simple and just use the 8GL Action Packer method to create a flat area and then use backpacking sleeping pads. They're light and take up very little space when not in use. A quite different solution to the same goal of comfortable shelter.
Based on *my* preferences, I wouldn't care for a setup like the one linked above, because:
* I like headroom for sitting up and changing... low ceiling would be too claustrophobic.
* I like easy ingress / egress either via the side door or rear hatch.
* When driving I prefer an unobstructed view out the back window.
* I don't want to cook immediately next to my rig and bedding, because odors and bears and fire safety.
* I wouldn't like having that big of a structure and mattress to store when not being used.
I think it's great that it works for someone else and they like it. It does look comfortable and practical for them.
Whatever works for your needs is best. Looking forward to seeing what you come up with.