To make you own lunch box gift I would suggest starting out at your local party store (Party City is awesome!) You can often find the same theme as the birthday party your attending. After you find the lunch box (or pail ect) they have all kinds of little things you can fill it with. When on the hunt for little items, I often go to the Target Dollar Spot, Dollar Tree, Toys R Us (candy section), Party City, and sometimes Wal-mart. Then add a cute little tag and some tissue paper and ta-da!
These boxes run about 5 each and I spend approx $1.00 on each filler item. Which means a box will run about $10.00-$15.00 each! Once again hope you enjoy and can use this idea in the future!