Semistable vector bundles elliptic curve

Question

Let $n=(r,d)$, r=r'n, d=d'n and $M(r,d)$ the moduli space of $S-$equivalence classes of semistable bundles of rank $r$ and degree $d$. How can I construct a finite morphism $M(r',d')^n\to M(r,d)$ equivariant for the simmetric group $S(n)$ and such that the induced map on the quotient
$$M(r',d')^n/ S(n) \to M(r,d)$$ bijects?