Handle for a tool head that uses an M5 thread. Has an indentation for an M5 nut on the inside. I use this as a handle for brass brushes, but it can be used in any case where you need to attach some sort of threaded tool head.

Print with the "fuzzy skin" option for good grip.

Onshape source files can be found here:
https://cad.onshape.com/documents/369fa1df3f3b526e70b60c07/w/5e1878e1281d843293db711f/e/15d95bac5e7016496fada879