What about a plate, with neat hole for cable, & then coverage over transom to cover existing hole. Then fix with rivets and silicon, flush off silicon inside smooth

Thanks Guys, what I ended up doing was using one of those stainless covers walu has, then cut two pieces of 4 mm rubber gasket rubber the same size as the profile of the stainless cover. I cut a cross in the middle of both pieces, then cut one out to the right and the other out to the right. Laminated both together with the cable running through the cross in the middle, then filled up the inside of the stainless cover with marine sealant. Seems to be nice and water proof and pretty tidy looking.