Here's the original fret dolly, circa 1970:
To make this tool, I made a little box of plywood, and lined with with aluminum foil. Then I melted some lead on my kitchen stove (toxic, but quick) and poured it into the mold while holding a bent steel bar in there to form the handle. Later, I added some shrink tubing to cover the 1/2" square steel handle.
In the years before I made this one, I used a simple block, hand-holding it under the portion of the fingerboard where I was tapping the frets. One fateful day, I was distracted and set the block down inside the guitar, came back later after forgetting it, and picked up the instrument, allowing the block to drop down and hit the side, cracking it slightly. NEVER wanting to do that again, I recast the lead block around a big iron handle, so I couldn't leave it inside anymore. I still hold it up against the fingerboard by lifting from the bottom.
Many years later, when I got into machining, I was doing some collaboration with LMI and designed and prototyped the fret dolly they have in the catalog. In our shop, we use them interchangeably.
It may be a matter of habit, but I prefer the heavy single block for its pinpoint accuracy of contact under the guitar top. A shot bag can do quite well, but I find it doesn't necessarily give support the way I like.
