You will need at least 6 packs (my unfolded milk pack size is 30x23cm(12x9in)). You also can use any thick flexible plastic packs or fused plastic.
2. Print and cut out the pattern from this pdf files one, two, three. Please use some used paper. This bucket pattern is without the seam allowance. Either your bucket will be smaller or add up half a cm (1/4 inch).
3. Transfer the pattern to the plastic. First the outer side (4 walls and bottom), you may want to make it look beautiful and include the best of the pack drawings into the outer wall pieces, as you gonna see outer side more often. After cut out the inner side from the plastic that is left. If there are not enough full size pieces, stitch some pieces together by hand or with the machine and after that cut out the pattern. You may also want a beautiful drawing on the inner bottom. I used a heart image from a fabric conditioner pack.
I sewn with the machine the inner side. Also possible to do it by hand. This material is a bit tricky and can slide to sides when you sew it. When I had my inner part ready, i put it aside.
4. I stitched manually outer part using the bias. First wall sides and then the bottom part. I used rather big needle and rather thick thread.
5. Cut out the sponge. I used it only for the walls. So that the spongeless bottom allows you to fold the bucket easily and keep it folded.
I stitched by hand 3-4 cm center of the sponge to the center of the inner wall. To stitch the walls of the sponge is unnecessary. When you put this inside, it will still keep the shape.
6. Put the two parts of the flexible milk pack bucket together (checked twice that that bottom angles and upper wall sides are matching (cut a bit where they exceed)) and stitch them. I started from the middle of the walls, so I can correct in the corners a little bit more to make "in and out wall seams" match perfectly.
7. Here we go, the bucket is ready and is used for keeping the stuff.