Really, the thicker the gel cushion, the better. You can get decent ones for $30, but "Purple" makes some thicker ones, and I ended up spending >$100 on one of their "double" thickness ones. Apparently now they've got an "ultimate" model that's even thicker, but costs like $160. Which is alot.
I stripped my old bog-standard "exec" office chair down to the bare wood, then sanded and tung oiled it and plopped a multi-foam pad on it (Muji, I believe). My butt refuses to be comfortable on anything else now.
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But even a thinner one like this is really quite decent:
https://www.amazon.com/dp/B0BGKQC3JH