The latest update to RSVPMaker gives you more flexibility for formatting email messages, including broadcast messages like this one and RSVP confirmation, notification, and reminder messages. You can take advantage of core WordPress formats as defined in the Gutenberg editor, and the CSS code will automatically be included in your messages as inline CSS (which works better with various email clients).
See below for examples and read the how-to blog post.
You can see a live example when you register for the RSVPMaker webinar.
For a cookbook of email template designs, see 3 Confirmation Message Design Examples. (If you're a professional designer with examples to add, I'll be happy to share them and credit you).