S.L.
Rather than invest in a mattress cover that you'll only need for a short period, consider using a "drawsheet". When I finally got my then 4-year-old out of pullups at night, I bought a couple of changing table pads for the middle of the bed. I folded a flat sheet in half and put it over the pads and tucked it in. On the rare occasions when she wet in the middle of the night, we just pulled off the pads and drawsheet so she could go back to sleep. We didn't need to use it for long, so I wasn't out the price of an expensive waterproof mattress. My daughter is quite the restless sleeper, but never "missed" the pads.
You can also buy a rubberized draw sheet at a medical supply store that sells supplies for the elderly. I just found the changing table pads to be softer and more absorbant.
HTH
S. Langmead