First of all I took careful measurements of the existing frame, then knowing that the rush cord is 5mm in diameter, I marked a pencil line around the perimeter at 5mm in from the edge. This was removed with the jig saw.
The cut was made on the outside of the pencil line, then I measured the frame again before planing the newly cut edges to a smooth surface.
Final job, rush the modified seat pad frame with paper fibre rush.
Hope the customer likes it!