I've worked with several stepper motor driver ICs that feature a ground pad on the bottom of the IC and recommend an unmasked copper pour connected to the ground pad via thermal vias, on the opposite side of the board from the IC, sized at least as big as the IC itself. Like, the manufacturer's suggestion is literally to use the copper as a heat sink. If you wanted, you could then affix dedicated heat transfer features, like a traditional finned heatsink, or a heat pipe to a dedicated cooler.