I updated arbcav to include more functionality. Now, in addition to giving you the transmission/reflection spectrum of the cavity you tell it to build, it will also calculate the HOM structure (carrier and sideband) for linear, triangular, and quadrangular cavities, taking astigmatism from arbitrary angles of incidence into account. You have the option of whether or not to include RoC for the mirrors. If you don't, it will just spit out the T/R spectrum as before; if you do, it will give the HOM info. You can also choose whether or not to give it a modulation frequency, and if you don't it will only plot the carrier HOMs.
I've updated the copy on the SVN (link above).
Below is the output of the function for our cavity. With the new length of 3 m (for FSR = 100 MHz), it looks like fmod = 19 MHz is the best bet.
I have made the HOM plot such that higher orders correspond to lighter shades (as well as shorter lines) to aid in identifying the stronger modes.