Method; cut four 20 x 15 cm sides and two 15 x 15 squares for top and bottom plus one 15 x 12 cm strip for the handle. Interface everything.
Create the handle by folding the sides of the strip to the middle then fold in half and top stitch closed. Top stitch the other side for effect. Stitch the handle to your top piece, fixing it in an ‘x’ pattern at either end for strength.
Line up the four sides inside out and stitch with an even seam width down the 20cm sides to create a cube (sort of) then with right sides still together together pin and then stitch the top into the cube.
Check you have caught all the corners then stitch the base piece in leaving a gap for turning.
Turn, stuff with wadding, gravel – best in a sandwich or simple mini sack – Try to surround the sack with wadding so the cube sits right and stuff more than you think necessary, then slip stitch the gap closed.