I also use a lot of glove leather to line the bottoms of my boxes. You can find some really good 12"x12", 18"x18", 20"x20" and similar sized pieces on ebay in a range of colors and textures. Personally, I use the full-grain King glove leather from PeggySueAlsoLeather. It is about 3 ounce weight and maybe 2mm thick. I cut it to size, test fit and trim, and then glue it inside the box using regular wood glue. Don't use so much that it oozes out. Then I use a block of wood to press the leather down and adhere it to the bottom nice and tight.