um, yeah...
I guess a color solid is okay for captions/subtitles that are continuous, but what if they are more sporadic? If you're going to spend all that time blading the Solid, what's the point of using the converter script?
I would have thought Mark Spencer would have used a better way to add positioning and backgrounds to all the titles. I know he knows how.
An easier and faster way would be to create a Title in Motion that applies a (conforming) background to the text and a default position on the screen as well as default font and size (and possibly tracking) — call it "Basic Subtitle" — then after applying all the captions turned to titles in a project, Select all the titles and double click on the Basic Subtitle title from the Title browser — the storyline titles will ALL be changed according to the Basic Subtitle defaults. Then find the long lines and add returns as necessary (the backgrounds should automatically conform).
I've made this title available free here: https://fcpxtemplates.com/sdm_downloads/basic-subtitle/
(PS - it took all of 2 minutes to make the original version — i.e., less time than it takes to mess with the solid color generator. The added published parameters came after some consideration for the end user's experience and the easiest way to deal with customizing.)