It suits both my eTrex 30 and my Oregon 600.
I posted in another thread, but thought it might be a bit rude having hijacked that thread, so I repost here and edit out that thread.
If you have a mountain bike inner tube of at least 2.35" width, you can cut a section of the tubing to approximately 1.5" (4cms approx). This then stretches around the unit gives some extra protection.
The buttons are no longer visible, but the bumps of the buttons can be felt, and the buttons work fine through the casing.
The idea I originally got a few years ago from Miroslav's blog and his review on the eTrex 30 (http://www.takeadventure.com). He used a wheelbarrow inner tube of approx 60mm diameter unstretched. My rubber inner tube was around 50-55mm unstretched, but stretched it would fit the eTrex 30 nicely, and stretched more would just fit the Oregon 600, although it was quite hard to put it on.

Pros:
- Adds a cheap bumper protection for the sides of the GPS.
- For the Oregon, as the case stretches across the front, so putting the GPS down display first on a surface means the rubber can prevent scratches on the screen.
- It helps the USB port cover stay closed and the unit remain waterproof. I have occasionally had both the USB ports accidentally catch and slightly open, so no longer fully sealed and waterproof, but luckily I noticed this before it started raining.
Cons:
- It takes a bit longer to change the batteries out. The Oregon 600 especially which has a larger back cover means I have to fold/roll down the back cover out of the way to remove the battery compartment cover.
I have misplaced the two original inner tube cases made from mountain bike tubes of around 2.35" width but I have been trying some 50mm silicone tube casing recently. This is what it looks like:
Similar to the rubber inner tube, but I find the thinner silicon a little nicer to the touch. The silicone is easier to roll back to access the rear covers as well which you can see here: