Waterproof covering for moving a double bass?

I have to walk 40 minutes or so with my double bass most days during uni term time, I modified one of those warehouse karts so I can tie it to that and wheel it. I used to use a single bed mattress case which fitted over the top perfectly to stop it getting wet when it rains. Now apparently someone has thrown it away. I need to get hold of something to do the same job or another mattress cover as I can't afford to buy a waterproof case for it. Obviously this is a bit tricky, its pretty specific and it has to be really tough plastic.

Can anyone suggest anything else that might work, or have any ideas how I can get hold of another mattress cover? If anyones bought a single bed mattress in england recently I'll take the plastic case off your hands heh.

Cheers,
Oscar.