The easist way to make a mold from a plug is to use modeling clay, gelcoat and fiberglass. There are several posts of the internet on this subject, so I will just give a summary.

You are going to make two half molds. Lay your plug on some plywood. Place modeling clay around the entire perimeter of the plug from the plywood to the HALF line. The modeling clay supports the plug and delimits the half mold everything above the modeling clay will be your half mold.

Wax the top of your plug and spray it with gelcoat. You will need about 2 coats more than you think you do. You will not be waiting until the gelcoat hardens. Recoat as soon as it is firm enough to take a new coat. The gelcoat is just to give a slick smooth finish to the inside of the mold.

As soon as the gelcoat can support it start laying fiberglass/polyesther resien. You are not building a lightweight hull. You are building a heavy, sturdy, cheap mold. Solid glass needs to be 1/8 to ¼ inch thick depending on the size and shape of the mold. Personally I cut a lot of 1x6 inch strips and lay 2 layer of glass then a core material soaked in resien. 2 layers of glass, more core etc … until I think it is stiff enough. For cheap core I use the blue shop paper towels or the non woven material used for cheap car covers.

When it is cured pop the plug out and cut the excess material off so you have a half mold. Leave a little extra so you can match the two halves. Repeat the process for the other side.

For a rudder repair you can get away with only 1 mold half and eyeball the other side. I am repairing a pair of EPO rudders that were broken into pieces so I will build a new skin and use the old rudders as a core.