Here's a quick little DIY waterproof saddle cover for you.
Old vinyl raincoat
Elastic (1/4 inch)
Big safety pin
Lay your old raincoat out and place your saddle on it top down.
Trace around your saddle about 4 1/2 inch out from the sides (more at the back end if you have springs you want to cover).
Cut out and fold edges in about 1/2 inch, Sew down to make a casing for the elastic. Leave about a 2 inch open space at the back for threading your elastic through.
Measure around your saddle cover and cut a piece of elastic 2 inches shorter than your measurement.
Attach your safety pin to one end of the elastic and work it through the casing. Once through, sew the two ends together.
Your saddle cover should scrunch up like a shower cap now. All thats left is to sew up the hole! You can seal the seam with some wax if you want. I never did and have not had any problems with water seeping through.
I've been using this saddle cover for well over a year now. It's been through heavy rains, snows, sleet and freezing rain, summer heat and everything else that it can possibly be exposed to sitting outdoors all day. It's held up wonderfully!