ember-file-upload to S3 with serverless lambda to return presigned URL

I'm using the following upload component:

ember-file-upload

with Ember (obviously) which has S3 support but all example of signatures require a separate server. As my project is server-less and uses lambda/api gateway, I would like to create a lambda to do the job. I've searched everywhere but can't find a working example or clear enough information that would allow me to port it. Has anyone been successful in doing this?

There are lots of insecure examples with secrets on the client but the uploads must be secure for this project and are therefore unacceptable.