A 555 seems to be the wrong choice here – either you're building something open-loop, in which case you'll have no feedback and no real regulation, or you're building something closed loop, and then your 555 becomes nothing but an inelegant method to solve a small subproblem of the overal converter problem; you'd simply not do that.
Instead, you'd use one of the many (cheap!) existing buck converter / controller ICs. In your case, the low-power IRL2104 implies that even the very classic ICs will do that at a much lower cost, probability of failure, complexity.
In other words the question "how to build a buck converter with a 555" is as far as I can see answered by "use the 555 as decoration only, and design a proper buck converter without it", just as "how do I design an airplane with chocolate pudding" with "use the pudding elsewhere and design the aircraft without pudding. Pudding doesn't help when building aircraft."
Also, fully aware that you might be subject to availability restrictions, but last time I checked a classic MC34063 (or clone thereof) isn't any more high-tech than a CMOS 555.
To be absolutely clear: if you can buy more modern ICs, don't buy the MC34063. It's really ancient (1983), and voltage converters have progressed a lot in the last four decades; buy something modern, which will have better ripple and regulation, higher efficiency and need a smaller inductor. Using a low-tech MC34063 is just still better than trying to shoe-horn a 555 into the job of a converter controller.