How can I fill odd-sized spaces with fiberglass insulation batts?
A "bib system" is often used in these scenarios. A mesh is stapled to the underside of the joists, and fiberglass is blown into each cavity.

If you really want to use batts you'll need to support them somehow. You could install strips across the voids and lay the batts on them. You'll need to use several rows and cut to fit.