Hi Gammy LW,
That's not necessary:
Garmin wrote:Waterproof Yes (IPX7)
See
https://buy.garmin.com/en-US/US/on-the-trail/handhelds/etrex-30/prod87774.html. IPX7 means that it's waterproof against immersion up to 1 meter deep (see
http://en.wikipedia.org/wiki/IP_Code). I haven't put that to the test, but rain is not going to be a problem.
I've been carrying mine around in just a simple holster that I clip onto the shoulder strap of my backpack, and I've seen some rain in the Highlands - never a problem. I've also had it taped to the deck of my kayak, and that also has never been a problem.
Cheers,
Stefan