I have been frustrated with purchased fitted sheets! They are always a little too small for our king size mattress (even the deep pocket style). Well, I invested $15 in an inexpensive king size flat sheet. Undid the seam on the top of the sheet, pressed open. Measured the mattress (12" deep), cut each corner of the flat sheet (12" square). Bring the cutout pieces together at each corner & sew down to the hemline. Double stitch the seam for heavy use. Cut about 18 inches of elastic (1/4" wide because that is what I had handy). Undid the side hems about 13" on each side (except for the top as I had already undid the seam). Mark the center of the elastic, place at the corner hem seam (inside the seam allowance). Stretch elastic on each side of center of corner seam - Tack the elastic at each end, pin the center to the corner seam. Sew the hem seam where you opened the seam. Voila! You have a fitted king size sheet that fits perfectly.
Now I can invest in some higher thread count sheets and have great fitting fitted sheets.
I used the same format as I use for making crib size quilts.