It's pretty simple and free to submit your podcast to Stitcher.
Note: When you are submitting your podcast to some services they may tell you that your Acast Open RSS feed is not valid. This is because they are using an out-of-date validation tool (like podba.se or w3c) which does not recognize the new Apple Podcasts iOS tags.
If this happens, please tell them:
My RSS feed is perfectly valid, and conforms to the latest Apple Podcasts iOS standards. Please process my RSS feed as-is.