Step 1: Requirements and getting started
1 Mountain bike inner tube
Puncture repair kit (optional)
Measure the length of your keyboard and cut the inner tube slightly longer making sure you have the valve in the length of tube to be used, exact measurements aren't needed, I just laid the inner tube over the keyboard and then added a few inches.
Step 2: Sealing the ends
Clean the inside of the tube and run some glue from the puncture repair kit around the inside where the cable ties will grip it, to give a good seal. Personally I've not needed to do this as the friction seal from the cable ties sees to be doing just fine.