This is an add-on for the very affordable, low-duty Pittsburgh 4-in-1 Quick Change Multi Head Mallet from Harbor Freight Tools ( https://www.harborfreight.com/4-in-1-quick-change-multi-head-hammer-69093.html ).
I needed a way to store the extra heads. I've included the STL's and the project file. The project file has the print profile for this which works very well.
( It's a strong .2 profile with 6 walls, 5 top and 3 bottom shell layers. Brim and snug supports used as well and printed vertically. )

You'll need to cut the M8 x 1.25 threads into the holes with a tap or some other creative way..

Scuff up the yellow shaft and use CA Glue to attach the MalletBuddy™.