Upholstery time! This was not a lot of fun, but turned out alright. I don't think I like it long-term, but it will do for now. I started by getting some extra foam - high-density, soft upholstery foam - and adding some extra cush. Also, I used it to contour the passenger seat so it doesn't have that awkward upward sweep at the back.
I cut the fabric into two oversize pieces and started with the passenger seat. I used a staple gun for most of the work, along with upholstery tacks for the heavy duty bits.
The passenger seat actually turned out exactly how I hoped, but the front seat is pretty lumpy and wrinkled. In retrospect, it was going to turn out that way simply because of its concave shape. Maybe after sitting on it a bit, I can re-pull it for a closer fit.
In other news, I called up Trail Tech to see if I could get the right size temp sensor housing for my Vapor instrument cluster. Turns out that their office is only 15 minutes away from me! I had no idea. Anyway, they were super cool and let me swap it out for the right one, no questions asked. 10/10 would recommend.