For more rugged cordura case options, have a look for "Molle Strobe pouches". I've found some of these fit the etrex series nicely. The Molle Grenade pouches tend to be too roomy for the eTrexs. There are some makers who do specific molle GPS pouches like Platatac, but I've found their eTrex option was designed for the older taller eTrex, and when used with the eTrex 20/30 is a bit too big.
Also, if you use the eTrex in a case while the GPS is on, there is a chance that you get accidental joystick button presses - which in my case led to random waypoints, and sometimes deletion of waypoints while walking. I previously posted about my DIY case for the eTrex here
viewtopic.php?f=21&t=20683 and you can see my solution for preventing the unintended joystick button presses as well.