This thing is the handle for a precision jack'o'lantern cutting tool. For a blade, this uses one half of a scroll-saw blade. Because of the small kerf and shallow blade, this tool allows for greater presicison in cutting complex shapes.

The jack'o'lanterns in the images above were created by using a vinyl stencil (cut on a vinyl plotter) and this tool.

To assemble this tool, take one pin-end scroll saw blade and snip it in half. Print two of the included file. Stick the two halves around the pin in the blade, then fasten the whole thing together with zip ties.

It's not pretty or comfortable, but it works great! Feel free to design one that's more ergonomic.