Canary deployments for Lambda with CodeDeploy and CodePipeline

Question

I can't seem to set up a pipeline that uses CodeDeploy for a lambda function. I have tried:

1. Generating the appspec.yml file as part of a CodeBuild action. In this case CodePipeline zips the file, and CodeDeploy says it can't find the appspec file.

2. Generating the appspec.yml file manually (Lambda invocation). In this case CodeDeploy gives the error "The deployment specifies that the revision is a null file, but the revision provided is a zip file." (It is definitely not a zip file)

Has anyone succeeded in doing this?

Note: I know SAM apparently does this. But it would be difficult to introduce new tooling at this point...