Doing a Google search for blue prints for a Volkswagen (VW) bus will provide a bounty of viable options to explore. I lean towards the 1967 and earlier models as it was in 1968 that major changes occurred that removed the small roof windows down along the sides.
I must put up a huge disclaimer for all those folks that are absolute VW Bus fanatics. The depth of knowledge I have of this German marvel of engineering innovation is quite limited. My knowledge of the VW bus is mostly comprised of memories; some not so good memories and some really great ones.
I have a not so fond memory of a yellow one VW bus, probably a 1972 vintage, of which my mother would utilize to haul bales of hay and alfalfa for Arabian horses she had no business having in her possession. This was during a time frame before I moved to Hawaii. More warm and cozy memories developed after my move to the islands and even there I was more a driver than a crazed VW aficionado junky.
But if there ever was a path to becoming a VW aficionado it is the process of creating
THE MOKANA® ROKSTERZ™. The more I delve into the science of reverse engineering the limited blue prints available out on the net, and looking up close at the intricacies of this mechanical marvel, I am moving closer to being in the market as it were for a delivery windowless model. I have not yet been chased off at gun point by the owners of all the VW buses I have stopped at to take a look and to touch the paint and body structure. Maybe one day, when I see the one I have been looking for, I will have to offer the owner a bounty for his ride.
I do have one set of fond memories of when I was a kid regarding a delivery model. My oldest half brother had purchased a white windowless delivery model. He drove it up to Timberline Lodge on Mount Hood, 60 miles east of Sandy Oregon. It was there that the engine over heated and broke down. He made it home, recruited me; a non licensed 14-15 year old kid that had more courage than brains. We drove up to the lodge in his humongous Dodge Power wagon, latched a flimsy thread of a tow cable to the front of the VW bus, put me behind the wheel of this soon to be speeding bullet and off we went. The ride back home was extremely exciting as the distance between the back of the two ton power wagon and this speeding white bullet was about 15 feet. We made the trip in the middle of the night and I can still close my eyes and see the red tail lights of that awesome power wagon just off the nose of that VW bus.
Let’s get back to doing some reverse engineering.
If you look at this photo as I did (btw – I got this off the net and I would give attribution if I knew where the heck I had gotten it from), the one thing that was clear to me was that although some models look just like a brick with rounded corners, there is a definite RADIUS to the interior of this 1961 Volkswagen model.

So if there is a radius to the shell of this 1961 model how do we determine what that radius is? What would be really sweet is to have a set of blue prints with the dimensions on it. HAH! Like that is going to happen! But I did scour the net and I did find a set of very readable blues prints with limited engineering dimensions. You can find the same blue prints for free over at the :
THE-BLUEPRINTS.COM website.
Now that I have some baseline measurements, I was able to make a SWAG (Scientific Wild A?? Guess) at the radius and these are the steps I used to find the radius, or at least a radius that I felt would allow me to model and create the ROKSTERZ RIDE.
First take a look at the base line meassurement provided in this blue print:

One of the first things they drill into you while taking AutoCAD classes is that everything is drawn 1:1, that means draw it to real life meassurements. If the hieght of the 1961 VW bus is 1890 millimeters, then draw it to the actual hieght of 1890 millimeters, and that is exactly what I did. But I had to blow up the image to match the exact baseline meassurements and this is the front view of the 1961 model I took and enlarged from the base drawing:

I then plugged the above image into AutoCAD and using the arch tool and other techniques I was able to come up with pretty close to real life dimensions:

The left dimensions are from a base line created from the bottom intersection of the two circles. The right dimensions are the dimensions that match the technical drawings, i.e. 1890 millimeters is the hieght of the 1961 VW bus, from bottom of the wheels (848 mm up from baseline) to the top of the roof (2738 mm up from the baseline)
Here is what AutoCAD looked like with the full image of the FRONT VIEW plugged in:

Looking back at the image above of the interior, it appears to me that the radius is smooth and is in one complete piece. However, when you look at the technical drawing it appears as if the radius stops below the windows and then the windows go up at the angle indicated in my SWAG. This front view black and white picture seems to confirm this (I again apologize for the lack of attribution - if this is your picture - sorry):

When you look at this image of a
stolen 1965 VW recently recovered in the Spokane, Washington area after more than three decades, you can clearly see that the passenger door has a radius up to the window and then the window is angled up to the roof while the side barn doors have a complete radius from top to bottom.