Go to Walmart, or Target and buy a pack of cheap shower caps. They go on quickly and easily, and are cheap. I also have the cover made by Brooks, and use it for longer periods of time, but the shower cap goes on faster, and is more compact, making it easier to put on quickly when needed.
As for treating the leather saddle with Sno Seal or the like, don't. Use Proofide. If you treat it with a water proofer like Sno Seal you will ruin it for what it is good for, breathability. My Brooks is 13 years old and has been in all kinds of weather and it is perfect. MY brother still has a Brooks on a bike he bought used in the 70s, the Brooks came with the bike. The Brooks is still good, and I know he hasn't treated the saddle, just ridden it.