I'd like to make new covers for the table cushions, but not sure what the best choice of fabric would be. The ones on there now are removable (zip), so would like that for wash-ability. It appears the bottom is some sort of plastic, I'm guessing to prevent slipping around(?). What type of fabric is best for durability, can we use the same fabric for the top and bottom, or should we use some anti slide type material on the bottoms? And finally, if not able to make covers, is there a good place to buy replacements with a nice variety of sizes? Thank you!