Autodesk Fusion 360 a 3D C.A.D software is free for businessses that generate less than $100k per annum.
However if you want to do it yourself you’ll need to learn how to use autodesk fusion 360.
Whereas with physical prototyping you can use paper, cardboard, plastic sheets, clay, foam, wood etc.. to start mocking up various prototypes without much of a learning curve however this option is more time consuming in the long run.
if you learn autodesk fusion 360 you can make changes rapidly and with a few pushes begin 3d printing those changes for fast quick iteration
or hire a industrial design studio.