Try one of these in your current chair. The cutout is where your tailbone sits so there is no pressure on it.
Basics of cushions, just in case you weren't familiar. The round 'donut' type cushions are for your genitals. When you sit on them naturally they hit your coccyx because they aren't meant to help there. You want a 'U' shaped cushion that's open to the back with enough thickness and firm foam to support the area.
I have three cushions right now.
This one is in my wheelchair on top of a normal wheelchair cushion. Otherwise I don't think I'd be able to sit in it! Picked it up April 2018. I initially picked it up to upholster a dining room chair I could use without a cushion but found it was easier have a second once I couldn't drive anymore. And I'm really glad I have an extra with the stupid wheelchair, LOL.
I've had this one in my car since April 2017. It's rock hard initally during cold weather but squishes down to nothing during hot weather if left out. My suggestion would be to just being it inside with you if you're not sure. My heated seats don't seem to make a difference, it has to be HOT.
I picked up this one for the dining room chair also but returned it too because it wasn't firm enough. Might as well be sitting on the seat.
One is a folding type from TJ Maxx, so I don't have a link, might be this one? It was in the travel section so maybe you'll get lucky and find one if you need it. I haven't tried it yet (it was $5) because I'm rolling around with lots of cushions now, lol... But it seems like it's going to be enough for good days.
If you feel like you need more thickness under your cushion I would suggest buying interlocking foam mat tiles and cutting one to shape. Stick an anti slip rug underlay between them so you don't get hurt, I like the IKEA one but you can get them lots of places.
The foam tiles (or a firm thick yoga mat) also work in a pinch when you need something NOW. You'll want to glue layers together first with something like e6000 and wait until it's cured to cut the mat or tile. Long term it needs memory or upholstery foam over it.
We've also done custom chair modifications to lengthen the amount of time I was able to work, so PM me if you need any info on other seating. It really sucks to have to work when you feel like you got the shit kicked out of your hind end.
Best of luck. :)