function does not replicate \markup?

[address@hidden]

I have just made my first Lilypond function!
but somehow it doesn't do what I want it to do...
The box with the number isn't located at the same position and it's a 
bigger size.
WHY?

// Anders

%% Start of file
\version "2.14.2"

fN = #(define-music-function (parser location fretp ) (string?)

#{

\once \override Score . RehearsalMark #'direction = #DOWN

\once \override Score . RehearsalMark #'self-alignment-X = #RIGHT

\mark \markup { \box \italic  $fretp }

#}

)

{

<c' e' g' >_\markup { \box \italic { 8 } }

<c' e' g'> \fN "8"

}

%% End of file