Tom, welcome to PF!

You have obviously done your reading and understand what's necessary

If you plan on using the Gizzmo, plug the return line - you can use a rubber - expandable plug ( #9 would work well for this) or a threaded 1 1/2" plug (like a SP 1022 - C) to do the job.

Putting the Gizzmo in the skimmer says to me that you want to refill the pool with free rain water, you can't do that if the return line is open, it's lower than the skimmer and, unpluged, would just let any additional water added to the pool flow out of it & also, any water on top of the cover would push the pool water higher and flow out of the unpugged return.

It's always your choice on what and how to do things Just be sure to 'run' your plans by us to know that you'll be doing things the 'proper' way.