When making box calls you usually use the softer of your two woods for the box and harder woods for the lid. Good box woods that are easy to get would be red cedar, poplar and maybe butternut. Walnut usually makes a good lid on any of these woods. I also like cherry and purpleheart for lids. A walnut lid on a walnut box is also a good combonation. I have never used oak in my box calls so i really do not know how well it works. I have recently seen radiused lid material on ebay that may meet your needs. The quality of the materials will be a big factor in the quality of the look and sound of your box. The sound boards should be rounded over slightly and not left flat. As far as tounge oil on boxes, I never used it. I used spray can deft as it dries quickly and after 3-4 coats gives a good finish.