The Spec-D seats were nice and they fit, but one of them had a tear in the vinyl and they didn't provide enough head room for a helmet. I thought about using them and keeping the stock seats if I do a track day, but I ended up sending them back.
Instead, I ordered a Kirkey kart seat:
https://www.kirkeyracing.com/product/10/21500/SEAT-ALUMINUM-16-HIGH-BACK-KART. I test fitted it today - perfect! Plenty of head room, it's narrow enough to center on the steering wheel, and there's room in front of the seat to step into the car without stepping on the seat.
The seat covers are on back order, so I won't get them for a couple weeks. I'll post pics after I get them and get the seats installed.