Not sure what these other bidders do for a living? But I have been doing custom power design for over 10 years for a leader Semiconductor supplier. The guys that make the buck controllers/Regulators. These bidder maybe able to give you a paper design and sim, but when it does not work and you have stability issues, they will not quickly solve your problems you may very well encounter. Stability of the power supply is primarily dependent on compensating the loop, but it can also be effected by Input filter issues, or a poor pcb layout. The design will take me 1 hour to do, your request is a simple one with many options, depending or your needs in order of priority relating to efficiency, cost and size of which you have not disclosed what you needs are? The PCB layout is very critical of the power design. My experience is extensively involved with customer power design, PCB layout, Debug and test of bespoke requested to meet specifications. I have worked with all major manufacturers from consumer to telecom to Industrial and medical. Feel free to ask any questions you wish.
PS, I do not have experience in Simulink, But I can provide you with an excel spreadsheet calculator. Or I can provide you with Math cad if you have it? PS how can you really expect to get quality work at $30? seriously?