  We demonstrate an easy to manufacture, 25 mm long ultra-stable optical
reference cavity for transportable photonic microwave generation systems.
Employing a rigid holding geometry that is first-order insensitive to the
squeezing force and a cavity geometry that improves the thermal noise limit at
room temperature, we observe a laser phase noise that is nearly thermal noise
limited for three frequency decades (1 Hz to 1 kHz offset) and supports 10 GHz
generation with phase noise near -100 dBc/Hz at 1 Hz offset and <-173 dBc/Hz
for all offsets >600 Hz. The fractional frequency stability reaches
$2\times10^{-15}$ at 0.1 s of averaging.
