This isn't a perfect match for you, but it may be enough of a solution.
http://www.countycomm.com/carrycase.htm
Scroll to the bottom, you need the bigger one to fit an X50/51 Axim. It is big enough.
There is also a slit in the velcro clasp for wires/antenna. Also a slot for credit cards, drivers licence and such to work as a minimalist wallet.
The triple locking flaps in back fit on handlebars, pack straps, belts and is a very secure system.
It's made of ballistic nylon with a soft lining. As such, you can spray it with any of the water repellents or even a wash-in DWR. The sides offer less weather protection than the front and back
For you, the problem is no clear front or button access.
I have the small version now that worked great for my Sony Clie or cell phone. I have the bigger one coming this week. Can post pix then if interested.
Phil