Another thought - I do like flat beds (though I don't own one myself). But on those the whole bed is above the top of the wheels. That could mean that at its lowest point the truck bed is maybe up to a foot higher than the floor of a conventional pick-up load bed (or else you are limited on tyre size which could affect your offroading). That could mean your pop-up camper roof when in the down position is close to the hight of a hardside truck camper on a conventional pick-up. Something to consider if trying to keep the profile low for offroading.
Since you are considering commissioning a custom build you want to get it right first time. What about deferring the purchase of that for a year and in the mean time buying a cheap used pop-up camper on a regular truck and trying it out in all the types of terrain and climates you intend to explore? When you come to sell it to buy your 'keeper' you will be armed with a lot of experience of exactly what you want, where you want it in the camper, what is important to you and what isn't. Plus you'll know what breaks and where to demand extra reinforcing.
Steve.
