We present an almost fully analytical technique for computing Casimir interactions between periodic lamellar gratings based on a modal approach. Our method improves on previous work on Casimir modal approaches for nanostructures [Phys. Rev. A 82, 062111 (2010)] by using the exact form of the eigenvectors of such structures, and computing eigenvalues by solving numerically a simple transcendental equation. In some cases eigenvalues can be solved for exactly, such as the zero-frequency limit of gratings modeled by a Drude permittivity. Our technique also allows us to predict analytically the behavior of the Casimir interaction in limiting cases, such as the large-separation asymptotics. The method can be generalized to more complex grating structures and may provide a deeper understanding of the geometry-composition-temperature interplay in Casimir forces between nanostructures.