Hand screwing cap for M10 (17mm) head bolt.
In the model is 17.5mm hole - does not fit tight - I glued it with hot glue

For another head sizes, just rescale it in slicer:
22mm - 125,8%
21mm - 120%
20mm - 114,3%
19mm - 108,6%
18mm - 102,9%
17mm - 97,2%
16mm - 91,5%
15mm - 85,8%
14mm - 80%
13mm - 74,3%
12mm - 68,6%
11mm - 62,9%
10mm - 57,2%
9mm - 51,5%
8mm - 45,8%
7mm - 40%
6mm - 34,3%
5,5mm - 31,5%
5mm - 28,6%