That sounds right. I wanted to measure the hole in my FastFerment lid to answer this question, but haven't had a chance as it's currently in use lagering my latest brew.
The only challenge I see in a DIY lid is the "hollow" stopper. @ryancgoodfelllow, could measure the hole in the stopper? Or maybe Aaron will see this and respond with the stopper specs.
I'm interested in this myself as I may upgrade from the FastFerment to a Brewbucket in the near term.