  We present an interrogation laser system for a transportable strontium
lattice clock operating at 698 nm, which is based on an ultra-low-expansion
glass reference cavity. Transportability is achieved by implementing a rigid,
compact, and vibration insensitive mounting of the 12 cm-long reference cavity,
sustaining shocks of up to 50 g. The cavity is mounted at optimized support
points that independently constrain all degrees of freedom. This mounting
concept is especially beneficial for cavities with a ratio of length $L$ over
diameter $D$ $L/D>1$. Generally large $L$ helps to reduce thermal noise-induced
laser frequency instability while small $D$ leads to small cavity volume. The
frequency instability was evaluated, reaching its thermal noise floor of
$\mathrm{mod}\,\sigma_{\rm y} \approx 3 \times 10^{-16}$ for averaging times
between 0.5 s and 10 s. The laser system was successfully operated during
several field studies.
