got the idea from special (but expensive) pouches in SkyMall mag

Step 1: Materials

the materials you need are:
1.)an innertube from a bike
2.)Superglue
3.)a waterproof Ziploc baggie (test for waterproofness by filling it with water, sealing it, and turning it upside down)
4.)a phone
5.)scissors

Step 2: For Some Phones: Resizing the Innertube

Measure the phone up onto the innertube. Cut two or three sections one to two inches longer than the phone. Split the sections with scissors. Glue the sections together with superglue to form a loop. Pinch your nose, yell "N00B!", and run out of the room while simultaneously escaping the fumes and giving the glue time to dry.

Step 3: Sealing the Ends

Glue one end of the innertube ring. Repeat the end of step 2

Step 4: Sealing the Buisness End

take the waterproof ziploc and cut the sealing part off of the bag. Then cut that to about a half an inch larger than the open end of the innertube thing. then use superglue to seal the thing onto the tube.

Step 5: Test!

Put a rock in it and float it in a sink. You may notice I taped the end of it. It adds extra sealing to the tube. If the rock is wet, cut some more innertube and try again. If the finished tube is used, there may be a little black gunk on the edges of the cell. If that happens, just ignore it, or, if you want, try to clean it off. It's really a matter of personal preference.