Mike, had a date mixed up; my wife gave me the RT seat rain cover for Christmas 2017 and purchased it from my local dealership that has since been sold and closed (a real pain because they had an excellent service dept and very savvy techs), and paid $99.99. You can see a picture at this link:
https://can-am.brp.com/on-road/shop/spyder-accessories/covers.html. Scroll all the way down the left column and you'll find it just above the Freedom Trailer cover. The cover fits both OEM and Comfort Seats and has an integral waterproof zip out cover for the driver backrest. It is a snugly fitting cover that is easy to put on, I fit it from front to back because of the "ears" on the front of the driver seat and the hardest part is getting it fully around the very back of the seat, and it has heavy duty shock elastic cord all the way around the perimeter. I have used the cover in the various situations we experience riding, from putting it on simply to keep the seat covered overnight on tour to having to stop on the road and install it before riding into a rain storm as well as putting it on before a day's ride because I knew it was going to rain before I arrived at my destination. When riding on it I do not slip around at all on the seat. Matter of fact on a long tour day's ride it might be nice if one could slide just a bit on it but it is very waterproof and I wouldn't want anything to risk compromising its waterproofing.
As an aside, when I was looking at the rain cover or driver backrest, that my great bride gave me for my birthday in 2018, I don't remember which, I called Cheap Cycle Parts and was told that there are some Spyder parts that BRP does not allow them to sell.
Hope this helps and Happy New Year!