Well, I finally got around to making my own seat out of a block of minicell foam using advice from a few DIY videos:

https://www.youtube.com/watch?v=5b6x5M6Z-QM (By Nick!)

and

https://www.youtube.com/watch?v=_NpDpjbpeI0

I moved the seat back in my Borealis a good 3 inches which has made getting in and out SO much easier, and so far it doesn't seem to had any impact on how the boat handles. I've paddled the new seat now in 15-20 knots winds, calm days, lots of chop, etc. and she paddles, tracks, and turns great. The new seat is soooo much more comfortable too!

I would up using 5" minicell foam because when I measured I realized that the seat bottom and the seat back rest would fit perfectly together if I used the thicker foam. I'm still fine-tuning the seat for pressure points, etc, and once I'm done with that I'll sand it smoother than it is now.

Most of the videos for making a custom seat out of minicell foam show the Holy Galahad concave carving discs which cost about $80ea. I was able to find a very similar carbide carving disc at Harbor Freight for only $10, and it did a great job carving the seat out with my 4.5" grinder. So $10 for the carving disc and $20 for a scrap block of foam from our local kayak/canoe outfitter, and I'm good to go!