I think the Ultimate pillion (for Valk anyway) is the same seat, regardless of whether you order it with a Bigboy or Lowboy set.
I had very good luck with an Airhawk (air) cushion. It lets air get under your hiney.
I got it specifically for a chunk of concrete covered in leather made by Corbin. But I used it on my old Ultimate too. Not till I got a new Bigboy did I mostly stop using it (now it's only for hot weather).
(Like a camping air mattress) You do not fill it full, but there is a lot of adjustability from a third to three fifths full of air. On long days, I used to add or delete air at stops, just to change things up.
They come with tie downs, but the cover is very non sip on the bottom.
And it won't break the bank.
https://airhawk.net/product-category/seat-cushions/I have no experience with a pillion cushion. It's possible a small rider cushion might work better on the back seat than their pillion cushion, depending on seat and wife dimensions. It might be OK to overlap the pillion edge a little, but too much could cut off blood flow to legs.
Slap a tape measure on your pillion. (I don't recommend that with wives)
Speaking of that, some wives with short legs get sore butt as much from an overly long reach to her foot pegs than the seat. Many guys have gone aftermarket to raise the passenger foot rests.
