Libsyn has instructions for setting up a feed redirect on their support website. These instructions are transcribed below for simplicity.
If you're importing a podcast from Libsyn, you can set up a feed redirect by following these steps:
First, log into Libsyn.
Click the "Settings" on your podcast.
Click the "Redirects" menu item on the left sidebar.
Set the "Feed redirect URL" field to your Pinecast feed URL (you can find this on your podcast dashboard). If you have a Pinecast podcast website, you can additionally set the "Page redirect URL" field to your Pinecast podcast website, though this is optional.
Click Save in the bottom left corner.