The first step is a short but serious opening; not too personal:

Attn: Sir/Madam,

The opening sentence should also address trust, mentioning a brand name could be helpful:

In appreciation of your esteemed contact received through a reliable source, I wish to introduce myself. I am Mr. ... , a customs officer serving with the International Customs Service (ICS) service number 2731827.

After a strong presence in the openings phrase, you may slow done your tone, increase mystic ambience and get the reader involved. Dont give in on trust:

First and foremost, let me apologize for using this medium to contact you in a transaction of this magnitude. Be informed that I got your enviable contact through the Information Online Services, while searching for an honest and reliable individual or company who can be honest with me at the end.

After this introduction you have prepared the reader. Now mention this delicate issue. Do not forget important details:

However, in the last month, while my team were patrolling through the Sea-coasts, we ran into group of Smugglers in the region suspected to be members of Liberian Rebel Groups of Mr. Charles Taylor coming with some goods and cash for purchase of Arms and Ammunition. On our way approaching the smugglers, they escaped from their boats leaving their goods and few Boats behind and the cash. We confiscated the goods; some of the goods were sealed metal boxes containing Defaced United States Dollars of 100 Bill Notes.

You have got the readers' attention, now further implicate the reader in the story, he is all yours. Move your focus at the same time to a solution:

My team and I took the strong metal boxes with other goods to my office for further declarations and I deliberately kept one of the boxes behind and took it to my home, safely and successfully. Because since they are confiscated goods from Smugglers, the cash will be deposited to the government account. So, I see this as opportunity to invest for myself and my children, instead of allowing the whole money to be deposited in the government account.

Point taken. One step back and iterate on trust, mix this with the virtue of the reader:

However, due to my position, as custom officer and still in active duty, the laws and the civil service code of conducts does not allow me to keep this money in my personal account or even use it now.

The reader is now looking forward to a next step. The expectations are set! Now, deliver the bonus:

I am seeking for your honest and truthful co-operations and confidential partnership to assist me keep this money in your company's or individual account, pending when I will fly into your country for my shares and disbursements. The total contents of each box is Seventeen Million Five Hundred and Eight Thousand United State Dollars (USD$ 17,508,000)

Pull the reader to make a final move, make sure you signal enough space for further negotiation:

I have kept this box safe and secret waiting for you to confirm to me your interest and honest to keep this money for me. And for safe-keeping this money, I have decided that your share will be 10% of the total sum.

Close the loop as you started the letter, with a professional closing, emphasise again that this is a personal engagement and do not forget to press on urgency:

Kindly, contact me urgently on my private email address (...) whether you are interested or not. And please, do keep this business confidential, as I would not entertain any form of exposure, as my other team members and the office are not aware that I have kept this box behind, if not they would have reported me to the authorities.

Convince by repeating what you have already said and take the opportunity to mention the urgency again:

Finally, due to time constraints, I will appreciate your kind and urgent response.

Close as you began the letter. The work is done. Well done!

Personal regards, Mr. ...