The plug on Amazon looks like the right shape. There are also repros of OE plugs with tabs:
https://www.chicagomusclecarparts.com/products/918731
Urethane is a great adhesive and it's good at filling gaps. Once you open the tube it begins to cure and it can only be stored for a short time before the entire tube is cured. I'd look for smaller tubes from one of the big box stores. Even better, they're available in different colors.