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The issue: need custom shaped covers for some plugs.
The solution: Hot glue custom molded to fit in place.

This is a bit janky, especially the first time, but with practice this seems to be a nifty solution. Obviously it's no good for hot environments or super strength but it's oh so easy, and that's a good thing.

Step 1: Needful Thingies

Tools:
Glue gun
scissors or knife
love

Materials:
Glue gun ammo
Cardboard (to form your outer mould)
Cooking oil (or other release agent that glue won't stick to)
Tape
More love (things are going to get sticky)

Step 2: Mold Release (nonstick Coating)

Dab cooking oil on the part you want to mold around.

This way the hot glue will harden around the object without adhering to it.

Generally I put enough on so the surface is well coated but not drippy.

Step 3: Outer Mold

Form cardboard into the outer mould shape you desire.

You may want to keep your mold fairly open because you'll need to access every crack and crevice with the nozzle of your glue gun... pretty intimate stuff.

The top can be a separate piece that pops on after the glue is injected but before it hardens.

Step 4: Injecto

Gluegun the mold full of hot glue. Start in the corners and under any protruberances. These were places I had to touch up later.

Fill the mold completely and put on the top if you have one.

Do any final adjustments to the outer mold shape if required. Then let it cool completely.

Step 5: Open and Adjust

After cooling, peel off the cardboard.

Your newly born piece will be adhered to it's mate. Mold release helps reduce this but there may still be stuck spots. Muscle and pry the piece off.

There may be voids. If so, re-oil your mould, pop some more glue on the void and put it back in the mold. This may or may not turn out well.

My piece had lots of protruding metal pins that loved to trap air pockets so I had to squirt glue right under them.