  We present a transportable ultra-stable clock laser system based on a
Fabry-P\'erot cavity with crystalline Al$_{0.92}$Ga$_{0.08}$As/GaAs mirror
coatings, fused silica (FS) mirror substrates and a 20~cm-long ultra-low
expansion (ULE\textsuperscript{\textregistered}) glass spacer with a predicted
thermal noise floor of $\mathrm{mod}\,\sigma_\mathrm{y} = 7 \times 10^{-17}$ in
modified Allan deviation at one second averaging time. The cavity has a
cylindrical shape and is mounted at ten points. Its measured sensitivity of the
fractional frequency to acceleration for the three Cartesian directions are
$2(1) \times 10^{-12}$/(ms$^{-2}$), $3(3) \times 10^{-12}$/(ms$^{-2}$) and
$3(1) \times 10^{-12}$/(ms$^{-2}$), which belong to the lowest acceleration
sensitivities published for transportable systems. The laser system's
instability reaches down to $\mathrm{mod}\,\sigma_\mathrm{y} = 1.6 \times
10^{-16}$
