On the V21 I don't see how you could put it on the inside. My dad's Mckeecraft I've always put it on the inside because I can. Pretty sure I did the same on my old beater boat back in the day. My V has the garboard style.

screw in from the outside

The only way on a V-21 from the inside is with 8' arm's so outside for me. I wouldn't want to use a compression style from the inside incase the pressure blows it out and a guy get's that sinking feeling.

On smaller jon boats w/no pumps, I plug from the inside so can pull it out and drain on plane...bigger boats w/pumps, I'm kind of an outside guy...