I would probably cut up some of the 1 inch foam boards from Home Depot and lay them on the floor under the cabinets with the reflective side up if I needed more insulation.
There is another alternative. I added 2" foam to the flat sections and since they are under the floor above they likely have some benefit in cold weather.
They're pretty easy to fit. Mine almost stayed in place by themselves but I still put some adhesive on them. And being a belt and braces kind of guy also put longitudinal light aluminum angles as back up for the glue.
I painted them black for appearance sake and also to add protection.
They've held up very well.