Here is the kit I use to add drains to my boat. It was just thin aluminum, so this kit worked best:
https://www.ebay.com/itm/375217811973
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You put the flare part on the inside of the boat, then the spacer, then you use the flaring tool:
https://www.ebay.com/itm/1-Drain-Tu...rine-Plumbing-Replaces-520290-1-/375541547600
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You really need the spacer to give the drain tube some depth for the plug to grab onto.
Also, I have found that the tool works best if you lightly grease all components, including the dies. I also add permanent sealant, as mentioned above, as I don't trust a rubber O-ring for long term service.
Once you have the kit, and it's cheap enough, you can add drains to anything, like coolers, livewells, fiberglass boats, etc. You can buy the tube by the foot.
I also use flared tubes when going through bulkheads. Good for passing fuel lines, control wires, etc. through without risking having them get cut on the aluminum over time.
A relateive cheap but very effective and permanent solution.
I hope this helps.
