Your right for the air tubes and things that are not submerged.As for the submerged Rubber grommets are good( in fact they are the best!.) My methods are solely the cheapest way I personally have found with shit I have laying around. I have never had any break off and once its cured its harmless to plants. I used it on my DIY cloner
https://www.rollitup.org/dwc-bubbleponics/302908-d-i-y-cloner.html , it is working great so far. I have spent less then 100$ on my whole 15 plant perpetual grow so far, just using old shit I had.