It sounds like you want a tamper-evident security seal. There are many commercial offerings. You can look at tamper-evident tape, cable seals, padlock seals, and many other options. I recommend that you read background information from Argonne National Laboratory, which has done some of the best research on the security of these seals.
Let me warn you of some caveats. First, the effectiveness of the seal is highly dependent upon the procedure used for inspecting the seal. Second, most seals can be defeated without too much sophistication, so they provide only a low level of security. You are not going to find something that "proves in court" that the item was not opened.
I also recommend you read Security Seals On Voting Machines: A Case Study, by Andrew Appel. It is a case study that looks at how these seals have been used to protect voting machines. It has many great pictures of examples of seals that have been used in that industry, to give you a feeling for the range of options. It also gives some hard-earned lessons: for instance, it explains why seals have been ineffective at protecting voting systems, and (as a cautionary note) it talks about how easy it was for someone with no prior knowledge of the topic to work out on his own how to defeat the seals.
With all that, tamper-evident security seals might still meet your needs. But you need to understand that they are not magic, they are not a silver bullet, and they do have some limitations.