How can I integrate S3 with SendOwl?

The instructions for doing so indicate that I needed to Visit Amazon S3 and generate a new Access Key and Secret, which I've done. Unfortunately, this has not worked. I get the error message, "The authorization mechanism you have provided is not supported. Please use AWS4-HMAC-SHA256." when I attempt to download the file, and I suspect it might be a permissions issue. Further clarification from SendOwl's instructions:

We currently use the v2 signing method, so your files will need to be stored in certain AWS regions for this to work. After doing a bit of digging around, I believe that my region, US-East (Ohio), meets the criteria, so I can only imagine that I have failed to set the proper permissions in S3 into place.