For the most historically accurate shield, you will want to use lindenwood/lime wood planks that are glued together, preferably a period glue (cheese glue is very easy to make and a very strong wood glue). The rounded shape is made by how the planks are joined, rather than the whole shield being pressed or steamed. Also, you would want to cover it in leather and edge it with rawhide. However, this can get rather pricey. Gaffer's shields are a great option if you're on a budget. Covering the shield with canvas is an inexpensive alternative to leather and you can usually get a canvas painters' tarp at a harware store for cheap.