We have a plastic tank made of a lower tank and upper tank that are joined via a. 3" 90 degree. Something has fouled the bottom exit pump out path. It will take forever to remove and re plumb all the fittings. I want to cut an access hole but concerned with being able to properly seal a plastic tank. Keep in mind the access needs to be in the lower tank which will have positive pressure once it fills and starts filling up the upper tank. Suggestions on how to seal up such an access hole? On a time crunch to get this fixed before boat gets loaded.