Attach the foil to the box so the shiny side faces out. The more light the foil reflects, the less heat the box will absorb. Smooth the foil after you apply it, so that it wouldn’t be wrinkling too much. If you’re using a cardboard box, glue foil onto the inside, too. Use a single sheet of foil large enough to cover the entire interior. Carefully mold the foil into the corners, and try not to tear it. Lining the inside with foil can help keep the cardboard from getting soggy.

Avoid covering the lip of the lid with insulating material, and make sure it can still fit over the box.

The box should keep your items cold for about 4 hours. If possible, keep the box away from direct light and sources of heat.

For example, if you use a 24 by 24 in (61 by 61 cm) box, the space available for storage will be 18 by 18 in (46 by 46 cm). A cardboard box is the easiest choice, but a large plastic tote would work. You could also make your own box out of wood. [5] X Research source

Duct tape the bag to the top of the box; add strips of tape along the box’s entire top edge. Give the bag some slack so there’s extra material at the bottom corners. It’ll tear easily if it’s too tight. The waterproof layer will help keep melted ice or condensation from getting the cardboard soggy. A trash bag is inexpensive and readily available, but a shower curtain or plastic tablecloth would be sturdier.

Cut 2 of the side foam panels 2 in (5. 1 cm) shorter to account for the thickness of the other 2 panels. Suppose you have a 24 by 24 by 24 in (61 by 61 by 61 cm) box; make 2 of the panels 24 in (61 cm) long. Since the panels are each 1 in (2. 5 cm) thick, make the other 2 panels 22 in (56 cm). Cut with the grain of the foam to keep the edges straighter.

Once they’re glued into place, the tops of the side panels should be 1 in (2. 5 cm) below the box’s top edge. Each side is 2 in (5. 1 cm) shorter than the box, but the bottom foam panel adds an extra 1 in (2. 5 cm) to their height. To make the glue dry faster, don’t use it in excess and make sure to apply it onto the dry surface.

Glue the 4 panels together to make an inner box. This smaller inner box is actually the storage space. You’ll fill the gap between the inner foam box and the panels that line the cardboard box with insulation. All of these layers of insulating material will help keep the cooler nice and cold.

In a pinch, fill the gap with packing peanuts, bubble wrap, or Styrofoam. Fiberglass insulation would also work.

Cut 2 of the strips, then set them over the gaps on parallel sides of the box. Measure the length between the 2 covers, then cut 2 more strips to match that length. Be sure to use 1⁄2 in (1. 3 cm) thick foam board for the strips to leave room for the box’s lid.

Remember to place the longer cover strips across from each other.

If you’d like, glue handles or knobs to the top of the lid so it’s easier to remove. If you made a wooden box, you could make a wooden, foam-lined lid and join it to the box with a hinge. [15] X Research source

Laminated cotton, PUL lining, and vinyl are your best options for waterproof materials. If you want to stick with recycled materials, you could use a plastic tablecloth or a plastic shower curtain. Thermal batting, which you can find at a craft or fabric store, is the best insulating material. If you’re on a budget, you can recycle thin flexible foam or bubble wrap. Thin craft foam or packing foam might work, but the lunch bag will be stiffer. Pick a durable, easy-to-clean material for the outer layer, like canvas or denim.

You should have 1 large and 2 smaller rectangles made of each of the 3 materials, or 9 total rectangles.

Repeat the steps to baste the other 2 sets of fabric and batting rectangles. Look for spray adhesive online or at craft and fabric stores. Spray basting is the easiest method of attaching the batting to the outer fabric. You could also hold them together by pinning them 1⁄4 in (0. 64 cm) from the edges.

Weave the pins into the fabric rectangles 1⁄4 inch (0. 64 cm) from the edges. Make sure the long and short sides of the rectangles are aligned. Position the rectangles so their short sides are at the top and bottom and the long sides are at the left and right.

Use this 1⁄2 in (1. 3 cm) seam allowance, or the distance between the stitch line and the edge of the fabric, for all of this project’s stitches.

Remember to use a seam allowance of 1⁄2 in (1. 3 cm).

Repeat the steps to line the right panel’s top left corner with the main panel’s bottom right corner.

You should now have a rough bag shape with an open top and unstitched bottom. The long sides of both side panels should now be entirely stitched to the main panel.

All of the bag’s edges should now be stitched except for its top opening.

Finish by stitching around the bottom edges of the waterproof lining to flatten the bottom.

The two pieces should fit together snugly. Line up all 4 edges around the opening, and make sure the stitched edges of the side panels are aligned.

Again, use a 1⁄2 in (1. 3 cm) seam allowance. Overlap your beginning and ending stitches to help secure the thread.

The right side of the outer cover should now be visible from the outside. When you peek inside the cooler, you should be able to see the waterproof layer.

This last stitch will help stiffen the opening and hold the lining and outer fabric together.

There you have it! Enjoy your new handmade insulated lunch bag!