I've put over 1,000 hours on my
B7800, in which case I think I have enough seat time to comment on this...
I had this very problem with the factory seat. Put up with it until the rails finally broke. I bought a replacement seat from NorthernTool (about $100 at the time). I've likely put at least about 400 hours on it and can state that it's FAR superior to the stock and that you really don't slip off. Note that I don't have a lot of slopes, so can't really comment on how that works out: but one should be wearing a seatbelt in such situations, so slipping around in the seat is less likely (the seatbelt, however is another thing I have a beef with- it readily tensions and after a while you find that you're being over-cinched and have to release the belt and refasten).