I did this with my mega cart/atty. It took me 3 tries to get a plug, ruined a cart but finally got a good plug.
Then I use some silicone around the edges to fix the leaking and the whole "plug bends down too easy" problems. Let the silicone cure.
Mine tends to get a bit dry now and then, but all I need to do is point it down, thump it lightly against something (ie: arm of chair) it wicks in some more.
I personally liked the SFY mod more, as the thicker plug is easer to keep from leaking IMO, but making that out of silicone would be a major PITA.
Has anyone tried making a cutter so that it'd be possible to make these from say, sheet silicone?