A TypeVarTuple has an implicit upper bound of tuple[object, ...]. There’s currently no way to specify a custom upper bound. There have been some early discussions about adding this capability, but the rules have not yet been worked out. Similarly, a ParamSpec currently always has an implicit upper bound of Callable[..., Any].
Do you have a particular use case in mind? Perhaps you could post a minimal code sample that demonstrates what you’re trying to do. There may be ways to accommodate your use case without the need for a custom upper bound.