As someone who's done over a dozen fixed-fee jobs - DON'T DO IT. There are too many unknowns, even in "simple" jobs. It won't be worth your time and you'll regret it. In order to make fixed-fee contracts worth it I had to cut corners. I hated doing that, but I preferred it to dropping the contract altogether (sunk costs fallacy).

I've also found the clients who insist on fixed-fee contracts to be very demanding and petty. They tended to interpret the specs liberally. It's not worth the hassle. Trust me.