This adapter makes it easy to clamp rods down to a flat surface with a bar clamp.
It's parametric, and you can open it using Customizer to configure it.
In the photo, I have the bar clamped onto a counter top. I don't recommend this at all, because the threads will dig into the material it's on top of. Instead, put the whole rod on top of another board before you clamp it down. This will support the rod all the way to the end where you're cutting.
Choose some values in the "User-defined values" section if necessary, render, and print. Print with several walls and higher infill for added strength.