[prototype development]
You have a specification, and you have a clear idea of what you’re looking for.
The next step is to put some work into giving you a tangible view of your idea.
You need a prototype.
This may be a 3D printed mock-up of your product, or a working model that looks almost exactly like a finished article.
It will give you a very clear idea of what the ultimate product will look like and will be something you can touch, feel and handle.