I don’t like the push-in lid either. I have to hold it down with a strap. You can drill a hole in your original lid and install thermowell with a stopper like below. Hardest part for me was drilling hole in the stopper.
I did 4oz whole beans in primary after fermentation, like a dry hop, for 10 days. Just opened up and dumped them in. I was happy with the result however I might up it to 6oz next time.