There are two SHA-256 methods in Community Commons. Are you using the latest version of Community Commons? I think you need GenerateHMAC_SHA256 which generates the signature with a hex encoding.
I have implemented AWS v4 signing in one of my apps the past two weeks. This functionality is mostly implemented (in Java) in the AWS IoT Connector. This module also has a method which creates the relevant SHA-256 string, e.g. for S3's
I was required to make a number of changes to the code (e.g. to add custom headers) to make sure the signing works in more cases than the module supports, but they weren't very hard.
I think I actually found what Staples calls the easy button. The end goal was to upload data to an S3 bucket and when I started on this project, I found that I needed to be able to create an AWS Signature and so I was figuring out how to do that. Since it seemed like it was getting unusually hard to do in Mendix, I got some more information about how we were uploading data to our S3 bucket, long story short, found that what I really needed was the AWS S3 Connector and it will set that up for me.