To avoid all trial-and-error to figure out the relevant parameters to
\raise, I would propose the following alternative (which also gets rid
of the extra horizontal spacing between the parenthesis and the
accidental):
^\markup { \smaller \concat { \vcenter ( \vcenter \smaller \musicglyph
#"accidentals.natural" \vcenter ) } }
In fact, you can simplify it even further into the following, which
still gives the same layout:
^\markup { \smaller \concat { \vcenter ( \smaller \natural \vcenter ) } }
/Mats