Firstly on clay, Clay deposits in the lows of the soil layers, if you want to find clay you have to dig through the topsoil, which can be deep,shallow, or non existent. The clay may also be deep, shallow, non existent, but that doesn't mean it isn't 20 cm's away or a meter etc. If not go to a river and dig around, clay is every where.. The refractory capability of clays is your next concern, but not really a big concern, the lighter the color of the clay the more refractory, but as you said its not going to be used too often, so build it, use it, look at lining did it check or crack etc The sand you use, I would go with a coarser sand, river sand is good and adds to refractory you could also add some finer beach sand to the final lining but wash it first. The straw, I would leave the straw out totally, better to use just sand at 80% clay 20% maybe mould bricks, build over a wooden form, and burn the form out when fully built and dry, or mould your oven again a form but if you want a dome??? Do it all on site. Reasoning straw is proven to cause more cracking than it is believed to prevent, sand will not dramatically change volume I.e. expand / contract so less cracking. Inner lining, using same clay + sand + perlite or similar mix up a mortar that when your oven is built dried and the form is burnt out render the inner wall, this will give your strong she'll a softer but higher refractory lining. This is cheaper than tubs of fireclay I.e. the commercial render you use to repair fire places, and will definitely do for a pizza oven. Refractory is the ability for a material to reflect heat, keep its strength and shape, so it will keep the heat in the oven space. Pm if you want more details.