A model for S-wave $etapi$ scattering is proposed which could be realistic in an energy range from threshold up to above one GeV, where inelasticity is dominated by the $Kbar{K}$ channel. The $T$-matrix, satisfying two-channel unitarity, is given in a form which matches the chiral expansion results at order $p^4$ exactly for the $etapitoetapi$, $etapito Kbar{K}$ amplitudes and approximately for $Kbar{K}to Kbar{K}$. It contains six phenomenological parameters. Asymptotic conditions are imposed which ensure a minimal solution of the Muskhelishvili-Omn`es problem, thus allowing to compute the $etapi$ and $Kbar{K}$ form factor matrix elements of the $I=1$ scalar current from the $T$-matrix. The phenomenological parameters are determined such as to reproduce the experimental properties of the $a_0(980)$, $a_0(1450)$ resonances, as well as the chiral results of the $etapi$ and $Kbar{K}$ scalar radii which are predicted to be remarkably small at $O(p^4)$. This $T$-matrix model could be used for a unified treatment of the $etapi$ final-state interaction problem in processes such as $etato eta pipi$, $phitoetapigamma$, or the $etapi$ initial-state interaction in $etato3pi$.
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