Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4877584.

The local big box craft store only has 12.7mm neodymium magnets.

I sized the hammer up slightly and enlarged the magnet inset to 13mm x 3mm.

I rotated the hammer so that it would not need supports.

For me the magnet fit in the recess with pressure alone.