The pattern is very very simple. Basically, all you need to do is knit a rectangle and sew up the seams. :)
the sack ready to have it's ends sewn up
This is the pattern I used to fit the sunlight soap. Cast on 22 stitches with 5.5mm needles. Work in basic knit for 26 rows. Sew up side seam, turn right side out and insert the soap. Sew up end seams and there you have it!
the old used sack