Hi folks. I'm about to install new cabinets and we'll have the central heat coming through the toekick via 2.5" x 14" metal ducts (see image).

I've done the ductwork and closed up the subfloor, but there are some holes in the floor (a box about about 6x6" mostly under the duct and the irregular cutout for the ducting and water pipe behind it). They will be covered up by the cabinets, but I'm wondering if I should still close them up and if so, what method you would recommend. Cutting plywood to fit would be a challenge and securing it in the hole would be even harder. But duct tape seems like a pretty terrible idea. Is there some material I could lay down over the holes and staple?