Are there any manufacturers who custom build Truck Campers?

ACEI, seems you're just fishing for ideas. The best way to get acquainted with the TC paradigm is to crawl around a few on dealers' lots. You will find out in a hurry if it's for you. Climb up the stairs and climb the big steps to the bed. Lay on the bed Sit around the dinette in every position. Sit on the pot, pull the shower curtain, reach for the wand and reach for the shampoo, soap, and toilet paper. Does everything work for you? Another way is to see what some of the folks on here have put together as a truck and camper. There is plenty of reading material on the good the bad and the ugly about truck camping on this very site, but you will need to do your own leg work if you are actually interested. Beware; it is a quantum drop in size and comfort from any kind of towed rig to the minuscule TC. A lot is packed in there and price per square foot is the highest.
As always, you need to tighten up and refine your 'must haves' to see if you are even in the ball park with a TC. Otherwise, it's all talk and conjecture. There is no 'best' camper for everyone. It is a long way from the Host Everest to the XP Camper, a long way.
jefe