I don't know what the "formula" is for bug jars, but looking at the picture it seems to me that the corners are each about 1/5 of the total width of the jar. So, for example, if your bug jar is 10" across (and maybe 25" tall, at a guess from the pic), then the corners are made by taking a 2" "bite" out of them. Using a 2.5" square, place it on the corner of the jar rectangle, and stitching diagonally across the small square. Do this in all 4 corners. Add a lid - 6"x2" finished for my 10" example.
If you really want it to match, then you need to scale up the jars on the front of your quilt. If you provide dimensions for those, I can help you with the math.