I’ve taken multiple approaches over the years. I’ve never gone to the lengths of entirely fabricating my own seats, though it’s certainly within the range of the average DIYer depending on how complicated you want to get.
Way back when, I had a pre-built seat from one of those ‘racing / cockpit’ seat-and-frame combos you can buy on the internet. It was a racing-style bucket seat and it was OK for a few years.
Then I bought a pair of old car seats, specifically from an SUV-style vehicle which had the capability to rotate through 180 degrees to face backwards. In my cockpit design at the time that was the way to get in. Those never really got used in anger, though, as that particular project died when I moved house and the seats were too bulky and heavy to take with me.
I still had my old racing seat and I built a new base for that and used that free-standing for a while. Eventually I got rid of the seat and bought a gaming-style office chair that I thought looked nice, and I removed the seat pan from it and fastened that down to my DIY base, which by now was fixed in place. Then I rebuilt the base, adding a car seat adjustment rack so I could adjust it backwards and forwards. Lastly I added a kind of poor man’s J-rails that lets me move the seat left and right for clearance when I am getting into the seating position as otherwise it’s very, very tight with my pedestal.
When I build my new simpit I will build a new seat base and I may either re-use the current seat pan or look for something new. Because I’m not simulating any particular aircraft type I’m not worried about convincingly looking like an airliner style seat or a GA style seat etc. I just want it to be comfortable and fit me!