it seems you are bent to do the Brown Nassau. Great.
There is an important component to this type of slide rule, graph or whatchamacallit: the mechanics of the bearing that establishes the rotational axis - actually, its accuracy and stability. While you can use a pin for a while it will deteriorate quickly. May I offer some hints? Of course, there is a myriad of possible variations for this - you need to be flexible to what ever resources you have.
As you suggest, just build first a mathematically very simple - please not primitive - prototype for instance a device that finds the intersections of two curves that you know the result of beforehand. As a bearing, you might use a short 1/8" smooth steel pin with a thread inside. That thread will take later 2 stubby flat head machine screws with only a few turns each that will hold the whole thing together.
Next, get yourself 2 penny-size metal disks. I' recommend brass, copper I don't know but NO steel. You can find such disks on the www at jewelry outfits such as:
709 Sansom Street · Philadelphia PA 19106 · USA
Now comes the important part: Drilling precision holes through the centers of these disks. A regular handheld drill will not do! Best, ask a friendly machine shop. The goal are holes that are about 1/1000 to 2/1000 inches bigger than the above pin. For the shop, this job will be no difficulty at all. Remember, this tolerance is crucial for the proper function of your device!
Also drill holes through the scales at the center of the rotation axis - somewhat bigger than the above holes but at the same location. No particular precision necessary here.
The rest is just assembling things. The step to pay attention to is gluing the disks to the accurately centered scales. It helps if you included some fiducial marks and a set of concentric circles in your graphs. As a glue I use simply contact cement. Works great.
No further description necessary here - you did such things before for your Fullers.
Finally, insert the pin, put a simple washer on one side and insert the one of the flat head screws. On the other side apply 2 washers, first one neoprene washer ( see your hardware store) and secondly one regular metal washer. The neoprene washer will act as a break and as a spring. Then insert the second screw and tighten it until the scales slide firmly but without skipping. If needed add a washer or two to adjust the pressure.
Ready for testing- good luck!