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).
Does Northern Tool still offer that seat? Did the bolt holes line up or did you have to improvise?
I replaced the seat on my B2710 with a Northern Tool seat and it was a huge improvement. I think this is the same seat, or at least pretty close. I've had mine for about 10 years. It did require a bit of fabrication to get the bolt holes to work, I used slotted angle. It also doesn't tip forward, something I really miss because it had that little tool box under the seat that I can't use anymore. That also complicates using the PTO without being in the seat, like if you want to use a log splitter. I modified mine so the engine will still die if I'm not in the seat, but I left a little pigtail that I can use to jumper the safety out, so I can get out of the seat and the engine and PTO will work.
Looks like they do:
https://www.northerntool.com/shop/tools/product_200490177_200490177
I don't recall having to do any modifications. But, I seem to recall cussing a bit. So, if they're still the same, yes, the holes line up (but one has to play around a bit).
Pretty sure that you'll be happy with this seat. It's nothing special other than it just works (and it's cheap). The drain holes are a plus.