Out of the box, the Decent Tamping Cradle does not fully support the handle of a portafilter.   This severely limits the utility of the cradle for puck preparation, since you need an extra hand to hold the portafilter level.

This device slides into the end of the Tamping Cradle, and provides a rock-solid support for the handle of the Decent portafilter, allowing you to both prep your puck and tamp it in the same place.

I saw another solution on Printables for this problem, but it looked a little flimsy for my taste.

You can print it with pretty much any material or settings - use your own judgment on how hard you are on such things.  me, I like things like this strong, so I used PETG with something like 6 perimeters and a lots of infill.  I won't be breaking it :-)