Fabric406.com has this pattern for a 9" block. I don't think it would be too hard to make it into 12" block (just add more strips.)
https://fabric406.com/blogs/fabric40...nd-quilt-block. I would use the second method. I would make the straight sides of the triangle equal to 9". Once you get all 4 triangles sew together it should equal to 12.5" unfinished (maybe a hair bigger). Hope this helps anyone who wants to make this block.