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....I had acquired 70, 6mm plastic portholes and laid them on a sheet of plastic. I then mixed up some clear epoxy and spoon fed a drop into each hole and left overnight. Two days later, the stuff had not set, so I set about scraping it back out and ruined them, so they are now in the bin....
I've converted to brass portholes (in my mind), so will be sourcing them soon. When they are in it will be like you say, watertight. I have my kiddies paddling pool ready for more float tests.