Update 6/29/2022- I was able to do some testing. The mod worked quite well. I was able to get around 20 rounds in, that successfully ejected, before I had an unrelated issue with the barrel coming unglued. More testing will be done in the coming days.
Experimental and untested. This is intended to replace the rubber band with a spring. Assemble upper to lower receiver. Insert spring behind bolt and install plug into the lower receiver buffer tube threads.