God freely creates abstract objects.

Say a Libertarian free agent was offered the choice between getting a million dollars and being tortured for a week. They would always opt for the million dollars rather than being tortured because there is no reason to be tortured rather than to take the million dollars. So, under a good account of Libertarian free will, the absence of reasons to choose option A and the presence of reasons to choose option B will necessitate the agent's choosing option B. It is only when there are reasons both to choose A and to choose B that the agent's choice is indeterministic.

Let's apply this to any account where abstract objects depend on God's will. If He chooses to create the abstract objects, it seems like He doesn't have the free will to refrain from creating them (since He must create them in every possible world). But, if God necessarily has reasons to create these abstract objects and necessarily lacks reasons to refrain from creating them, then He will create them in every possible world. This is all totally compatible with His having Libertarian free will.