This is a basic case that can be done from household items.
You will need:
A mobile device (larger phones work better)
Step 1: Inflate a Balloon
Inflate your balloon but don't tie it off!
Step 2: Place Your Device
Place your device on top off the balloon and press hard enough to to keep it steady.
Step 3: Press Hard
Press hard on the device and slowly release the air in the balloon so the balloon wraps itself around the device.
Step 4: Release All the Air
Release all the air and rub out any wrinkles in the case. You're still able to use the buttons on the device. Once you've done these steps, simply cut off the saggy end and it's ready! If you wanted, you could add multiple balloons on the back and screen to water proof the device!