I have made a few but I have never seen anything marketed for the purpose. A 7/8" cup type welch plug should fit snug. (don't use the disc type) Install it with a thin film of Loc-tite #518 to ensure that it will never leak.
I like your boat! In answer,Iv'e used an expandable freeze plug before. Easy to install,remove and it is unaffected by the heat and trans fluid!