Many sorts of rain gear for cameras is on the market but much of it is expensive. Adorama has a good selection( search rain gear).   Perhaps the shower cap or seeing what is available and adapting a home made cover will do.

I have seen sports photogs with camera covers and apparently custom "tents" covering camera, tripod and photographer..then again, unless freelance someone else is paying for all of it and supplying the camera.  

Best wishes on a good wet day shot..I would like to have the same and get the exposure right to make it truly feel like the day actually is.                Tom.