Optimal shapes for positivity preserving

We are looking for an optimal convex domain on which the boundary value problem $$\left\{\begin{array}{cc}(-\Delta)^2 u_\gamma-\gamma\Delta u_\gamma = f,& \mbox{ in }\Omega\\ u_\gamma=\partial_\nu u_\gamma=0,& \mbox{ on }\partial\Omega\end{array}\right.$$ admits a nonnegative solution for the most $\gamma$, if $f$ is a given nonnegative function.