I've been able to alter the weighting of plastic baits by drilling a 1/4" hole in the back, removing or adding whatever weight I needed to, and then sealing the hole with bondo.
I spread the bondo about 1/8" thick onto the sticky side of a piece of blue painters masking tape, place the tape over the hole, and push it down so that the bondo that enters the hole is only that 1/8" thick+-.
Once it's set, I remove the tape, sand down to the bait's surface, and then use runny super glue to reinforce the bondo, and seal over the hole.
I've used this method to add weight to jerkbaits, and I've never had a problem, even though the balls in those baits move back and forth all the time.