The 2xHAT2165's means 2, HAT2165's in parallel. I think because they are surface mount package they are sharing the heat. You could choose a suitable single device. The 6 x 22uf are 6, 22uf caps in parallel to share the ripple current. You want a low ESR(equivalent series resistance) caps.
Sorry I don't have a drawing package handy, but you need a single supply op amp, a CA3140 should be fine. Connect Vsupply of the opamp to +12v, and V-ve to gnd. Connect R1(10k) to +24v(output) and to R2(4k7), other side of R2 to gnd. Connect the junction of R1/R2 to the + input of the opamp. Connect R3(10k) to +12v and to R4(4k7) and the other side of R4 to the output of the opamp. Connect the junction of R3/R4 to the -ve input of the opamp. This is the differencing opamp. You need a resistive divider on the output of the opamp R5/R6 whose junction you should connect to pin3(Vfb).The ratio will set the output voltage. The voltage at the output of the opamp should be (Vout - V12)/2. So the feedback ratio should be 1/4 of what is shown in the diagram. Roughly replace the 229k resistor with a 68k. But check my figures, because this is off the top of my head without a calculator, and it's not a good noggin. Use at your own risk.