You can secure your stockings with command hooks above the fireplace. Attach some greenery or décor pieces (like bells or ornaments) straight on the stocking or add some artificial pine stems above them.
When you don’t have a mantel focus on decorating the space around your fireplace. Place some blankets and pillows around, use décor pieces like baskets or and other decor pieces like miniature Christmas trees, stacks of boxes or gifts.