Fortunately, there is e.g. a free KiCad library with ready-to-use PCB antenna designs to help one get started with a custom design, as well as many other resources, covered in the article.

No user-serviceable parts inside? Give me a break! The head unit is tough to get to, but the PCBs within the chassis use ...

But there are also more serious concerns, such as the enforcement of design partitionings ... like the work of [Saar Drimer] and many others. But if one PCB doesn’t cut it for your needs ...