There are benefits to making both small and large housings for your scratch stock, including making multiples of each.
The small body is merely a block of wood with a kerf, threaded insert and thumbscrew, and it takes about half an hour to make. The small size fits comfortably in your hand and can be used with just one hand. You are, however, limited in the size of the profile you can make since the scratch stock is only supported on one end.
Conversely, the large body can hold a longer piece (larger profile) of scratch stock, because the "arm" supports it along its entire length. Two drawbacks: you need to use two hands with this housing, and the fence (the part that rides along the edge of your workpiece) is not as wide as the one on the small body. The larger body also takes a little more time to make, but is still very easy.
With both housings, you can alter the shape of the fence to match a set curve. You can also bevel the fence on the large body so you can follow any curvy edge.
Making sure the threaded insert is seated squarely in the small body is the trickiest part, but there are methods to ensure it's done accurately (search online for "installing threaded inserts"). I just used a T-wrench and went very slowly.
Now back to making multiples of each. Once your scratch stock is positioned perfectly in a housing, you don't want to remove it until your project is finished. But in the meantime, you might need to add a different profile with a different piece of scratch stock to another project. Rather than remove the original scratch stock and risk not being able to reposition it, just make another housing.
It's the same idea as using multiple routers while working on different jobs at the same time. And how many routers do we hybrid woodworkers have? Mine amount to all the digits on one of my hands. (Hint: I still have all my fingers. Knock on wood.)
If anyone finds an error in the illustrations, please let me know so I can correct it and repost. You can experiment with the position of the screws on the larger body— another arrangement might work better for you.