I gave away my commercial third hand as I did not like it. I wanted something that would work directly with my panavice. I tried using magnets for mounting these but even the strongest did not hold well enough so I 3d printed adapter blocks. The ones I have built so far just use clips on for the ends but this can be adapted to all kinds of assistive holding.
The rods are just coolant tubes from Amazon, if you need to do something more heavy duty just go up in size. The 1/4" work great for electronics work but I'd go to 1/2" to make a light or the like.