It's better to use width as parameter instead of outer radius, because then the constraint `r_out > r_in` (many fitters can't handle that) becomes simpler: `r_in > 0` and `width > 0` ... and the correlation between the fit parameters is smaller.

See here for an example: https://github.com/gammalib/gammalib/blob/devel/src/model/GModelSpatialRadialShell.cpp