I assume that your photo has been taken from an angle and all 4 of the "kites" that make up this fractional block are the same size?
If so
If you have a 45 degree kaleidoscope wedge ruler (several of them on the market), I believe you could cut 8 triangles of appropriate sizes to make the corner triangles. I believe you'd want them to each be the width of a side of the periwinkle plus a half inch, but don't quote me. Measure one of the periwinkle sides including seams and then experiment with a strip of fabric until you figure out what the width of the strip should be to create a 45 degree kaliedescope wedge with a "short side" of the same length of the periwinkle blade length.
Sew one side of two of the kaleidoscope wedge triangles together long sides together This will create a "corner piece" that you can then seam to one corner of the periwinkle. You'll need 4 of these per block. I hope that makes sense.
If you do it right, you could probably avoid sewing corner seams by sewing the short end of two of the kaleidoscope triangles to the periwinkle and then seam them together.