Understand, approx 70% of your projects effort will be in getting the mold right. If this is a one item project, you can cut a few corners, problem is, best if one has some experience first before deciding which corners to cut. Not sure if a gel coat is needed here, gel coat adds another complication/cost/step, and it mainly only adds to looks in finished project, and gel has a potential for "alligatoring", which defeats its whole purpose.

Some molds can be fudged as a single mold, by making the mold flexible and/or sacrificial/sectional, or in your case when the plug is the bumper or making the finished part in multi lay ups, a thin layup to be easy to remove from the mold, and then another after pulled from the mold to gain needed ,strength. Risky because you need to somehow maintain shape outside the mold for second layup.

It can be done, but its a learning process.