\documentclass{article}
\usepackage[paperheight=60mm,paperwidth=95mm, margin=5mm]{geometry}
\usepackage{feynmp-auto}
\addtolength{\oddsidemargin}{-10mm}
\addtolength{\evensidemargin}{-10mm}
% commas in feynmp sometimes lead to strange errors, use \comma instead
\DeclareMathSymbol{\comma}{\mathpunct}{letters}{"3B}

\begin{document}\LARGE
    \centering
    \begin{fmffile}{bsll_box_diagram} \centering 
        \begin{fmfgraph*}(300,130) 
            \fmfstraight   
            
            \fmfleft{l1,l2,l3,l4}
            \fmfright{r1,r2,r3,r4}     
              
            \fmf{phantom}{l4,q1,q2,r4}  
            \fmf{phantom}{l2,m1,m2,r2}            
            \fmf{phantom}{l1,p1,p2,p3,p4,r1}
            
            \fmffreeze
            
            \fmf{fermion,label=$b$,tension=10.0}{l4,q1}       
            \fmf{fermion,label=$u\comma c \comma t$,tension=10.0}{q1,q2}       
            \fmf{fermion,label=$s$,tension=10.0}{q2,r4}
            
            \fmf{photon,label=$W^+$,tension=1}{q1,m1}
            \fmf{photon,label=$W^-$,tension=1}{q2,m2}
                   %TODO!
            \fmf{fermion,label=$\nu$,tension=0.7}{m1,m2}
            \fmf{fermion,label=$\l^+$, label.side=left}{p3,m1}
            \fmf{fermion,label=$\l^-$}{m2,r1}
            
        \end{fmfgraph*}
    \end{fmffile}
    \pagestyle{empty}
\end{document}