With the large section of new flooring laid I set out to patch a floor air return that we no longer need.
A tip I knew about and used probably saved the job from having to be redone by someone with experience. I used a biscuit joiner to cut existing planks at locations I wanted to patch into. Here is a short video about the basic process from finehomebuilding.com:
After I made the cuts and removed the waste I added a new subfloor and now its ready for new planks.