You are looking for a home-made Wooden Box with a swing lid
The Cache is not on the Bridge but in a typical Cache location.
Please ensure when located you hide back well, obvious places to re-hide.
Don't want to give many other clues, but the green is well used by small muggles.
If you are venturing out at night please take a small powered torch as it can be dark in area but please note muggle houses nearby.