Truck camper

I would go the used route. Gut it and build up your way.

Check out the Alaskan campers.

Popups have big plus and big minuses. Research what owners say. I loved the mpg mine gave, but hated the canvas sides that made it a tent. I sold it after less than a year. For a couple hundred bucks (I do lots of miles) in fuel I can drive a full size one and not have the minuses.

Yes you can buy just the shell from Four Wheel Campers or All Terrain Campers.

Four Wheel
All Terrain

There is a couple of forums you can to and get ideas from others.

Expo Fourm
Wander The West Forum

Yes, Four Wheel Campers sells the shell. Here’s an example.

https://fourwheelcampers.com/model/hawk-model-pop-up-truck-camper-leader-full-size-trucks/hawk-shell-model/build

A couple builders offer just the shell. 4 Wheel campers has one as well as Northstar.

Never heard about anybody doing it, but for right "motivation" I guess any camper manufacturer will sell it to you.
Like the idea not only becouse of choice of floor plan, but also too many hidden defects come from original builders. :)