There is no commercially available product available to do what you request. The height of the overflow of bathtubs is pretty much standardized. that being said, the geometry of some tubs allows a “deeper soak” than others, especially some older cast iron antiques.
You can always cover the overflow with something like putty, and that will allow you to fill the tub more. However, you must understand that the purpose of the overflow is to prevent the tub from filling so much that it spills water onto the floor… By plugging up the overflow drain, you are bypassing an engineered safety feature of your fixture, so I cannot advise doing that. If you want more soaking depth, hire a plumber and have him replace your current tub with a model that provides you with what you want.
NtP