Did the same thing on my Pro-V, and learned a few valuable lessons.
They sell different sized plugs for almost everything, I’d highly suggest that route. Crafting your own customized version based on hardware store rubber stoppers, and other retrofits is just an exercise in frustration. It’ll get wet, eventually.
On mine, I had to plug the overflow vents, bottom fill, drain, and aeration units……all of which had different sized plugs! I had my boat at LaCannes marine anyway, so just kept going back and forth between the sales floor and my boat to find what I needed. I’d recommend dragging your boat to a local marine dealer to do the same. Some of those openings are tough to measure in there and be exact with. When in doubt, measure twice, and purchase once.
Something so easy sounding shouldn’t be that frustrating, but I think having it at a place that had alot of plug options was useful.
Joel