We build the sidecar around the donor motorcycle and the intended load. Mounting points, body size, entry height, seating, suspension, lighting and storage all change depending on who or what will be in the chair.
Bring your bike to Hua Hin for assessment, or send the model, year and clear photos if you are further away. For passenger work, include the approximate passenger size and whether easy entry, weather protection or additional storage matters.
If you only have an idea, that is enough to start. We would rather understand the use case first and then shape the sidecar around it.