I bought a couple of used hanging strops on ebay. One looks fairly old and the other looks like an almost new Dovo linen/leather strop. The leather is slightly turned up on the sides so that it forms a concave dish in the middle. I flattened it out with some books kind of but it's still somewhat uneven with some edge parts flat and other slightly raised still. Is this bad? Does the strop need to be perfectly flat? and if so does anyone have a cool trick to flatten it out better?