If a theory is committed to both universal causal determinism and incompatibilism, it appears that free will could no longer serve a theoretical role and would be an otiose assumption.
I recommend Martin Gerwin's book, Causes, Agents, Explanations, and Free Will, in which he develops a variant of incompatibilism ("soft incompatibilism") that accommodates free will to a limited extent. The ebook costs less than a cup of Starbuck's coffee: