https://www.ssa.gov/pubs/EN-05-10137.pdfThe above will give you some information on doing this. However, keep in mind that you will need to open a bank account in Belize and thanks to the USA this may not be too easy. You had better do this long before you decide to send SS checks to Belize.
An alternative would be to use an atm card drawn on an American Bank you have your funds sent to, I've done this several times. You could also, after you open a Belize bank account, write a check and deposit it to this account. There is almost a month before you will be able to draw on the check and the exact time depends on which bank you have an account in. In addition there are other ways to cash a check if you are known but I'm not sure of the legality. Many businessmen who need to buy goods and such in the USA need USD and have ways of depositing your check in a USA bank. They might cash your check, if they know you.
All in all banking and trying to do things the correct way in Belize is difficult and getting more difficult it seems. I recently closed my last account in Belize and when I did they ask me 3 times if I was sure that is what I wanted to do. They told me it would be very difficult to open another account and would require permission from the "central bank" (?) Because I'm spending less time in Belize and reporting foreign accounts is such a hassle it is just not worth it to me anymore. In addition I suspect that having a foreign bank account puts a big target on your back for an audit. I've read this several times. Please understand that I've not done anything to illegally avoid taxes or anything like that. Just had accounts to facilitate paying bills and having cash for living expenses without having large amounts of cash on my person or in my home.
Good luck.
Ambergris Caye 
AmbergrisCaye.com