I wish you success with your venture!
As another Callen owner I agree: a pop-top added to Callen-style camper would sell well.
There is a company in Chula Vista (south of San Diego) that adds pop-tops to Elements and Jeeps :
http://www.ursaminorvehicles.com/campers/element-camper/common-questions.html
I called them last year, to ask if they would consider modifying a Callen and adding a pop-top, but they are a small operation and it didn't go anywhere.
I love my Callen- found it used for $800 in excellent condition. I'm the 2nd owner, have driven it to Baja numerous times without problems. There are many all over San Diego- they probably will outlive the vehicles.
But I often miss the '71 Westies I sold for a song many years ago. Love that camper design, just wished they had more power!
If/when someone recreates that in a sturdy shell there will be a market.
As for why this design is called a "Baja camper"- I've driven with fiberglass shells on 4wd trucks in Baja. They flexed, the bolted-on roof racks shifted , and they couldn't carry much weight. If you are going to remote beaches you often need to pack in everything, so you need a strong frame. They were also low-profile and uncomfortable to use as campers if you wanted to sleep inside out of the wind/rain/dust.
The fiberglass shells also leaked "moon dust" which is what you find heading out on dirt roads to many of the Pacific beaches in Baja: Powdery, fine dust that gets into everything. (I suspect something like the SpaceKap could have similar problems.) The Callen is well sealed.
So: no dust leaks + metal frame that carries weight with no problem + comfortable interior space= Baja.
As for interior: they are empty shells.
These photos show mine at a whale watching camp (Scammons Lagoon) with a solar panel propped up on the roof- part of a GoalZero system to charge electronics.
I modified the interior: sleeping platform on left with storage below, lighter items secured above for travel, shelving on right for storage etc.
I sewed curtains and added deep pockets to store lightweight items that stay inside between trips.
The sleeping platform & shelving is easily removed/rearranged for times I need to carry large items.
