I simply had my doctor's office print a list of my prescribed medications. I carry the actual medications in their original pharmacy bottles.

So far so good. That said, Singapore customs has only stopped me once in the last decade on a couple of dozen entries into the country, land, sea, and air, and that one time the inspection never even got to my medications. They seemed to be more interested in the magazine I was carrying. When they found it was the airline inflight magazine they pretty much lost interest.

Best of luck.