Actually, I have seen a few homemade versions on the net. One was basically a very simple plunger style made out of pvc pipe and some sort of stuffing tube threaded on the end. The other was pvc as well, but they used a caulk gun. Both of them, were small, as I imagine they would have to be if you were solely relying on hand power to operate them, unlike a professional one where you have the difference in gear ratio working with you to increase overall force. I have thought about making my own from some stainless dairy equipment I have and a small hydraulic cylinder. Even thought it would be cheaper to just buy a good one, I love to tinker with stuff. If I do make it, I will for sure take photos and set by step pics of its constuction and post it.
I played with the idea for a long time. Was going to use large diameter PVC pipe, threaded rod, the same material as plastic cutting boards for the plunger, and build some sort of frame. In the end I bought 4 used F **** brand 20 liters, cleaned, polished and overhauled them. I sold three for enough more than I paid to pay for my one. Couldn't be happier with the stuffer.