A great holding device, especially for relief carving is a bench hook. Simple to make, I put this one together with scrap wood I had laying around the shop. You can build this in whatever size works best for you. Mine is 14 1/2" x 19" because that's the size of 3/8" plywood I had. I added 3/4" x 1 1/2" cleats on the bottom and 1 1/2" x 1 1/2" on top. It's called a bench hook because it hooks on the edge of your table or bench. You can then carve on your piece as it butts up to the top edge board. You can add non slip padding to it for better control.