That is a very nice bento box. Get a furoshiki and you can take that anywhere.
A furoshiki is the cloth you wrap around the outside of the bento box (or other boxes for presents/food/souvenirs) and tie in a knot at the top to keep the lid on. They are usually rectangular and range from what we would consider ordinary cloth napkins to extremely ornate and expensive brocades. Styles of knotting range from simple to decorative to knot-forms-its-own-handle-out-of-the-cloth. The way I usually do a furoshiki: get a square or rectangular piece of cloth strong enough to support the weight of the box and large enough to wrap around the box entirely. Place the box so that one corner of cloth comes up over approximately the middle of each side of the box. Tie one pair of opposite corners together middling tightly, using a square knot. Take one of the remaining free corners and insert it under the knot so that it comes out the other side; take the other free corner and do the same. Tie another square knot on top of the first one, as tight as you can get it. You can insert a carrying strap into this and it comes undone relatively easily, but it won't really go anywhere by itself.
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A furoshiki is the cloth you wrap around the outside of the bento box (or other boxes for presents/food/souvenirs) and tie in a knot at the top to keep the lid on. They are usually rectangular and range from what we would consider ordinary cloth napkins to extremely ornate and expensive brocades. Styles of knotting range from simple to decorative to knot-forms-its-own-handle-out-of-the-cloth. The way I usually do a furoshiki: get a square or rectangular piece of cloth strong enough to support the weight of the box and large enough to wrap around the box entirely. Place the box so that one corner of cloth comes up over approximately the middle of each side of the box. Tie one pair of opposite corners together middling tightly, using a square knot. Take one of the remaining free corners and insert it under the knot so that it comes out the other side; take the other free corner and do the same. Tie another square knot on top of the first one, as tight as you can get it. You can insert a carrying strap into this and it comes undone relatively easily, but it won't really go anywhere by itself.