I add BBs to some jerkbaits by drilling a 3/16" hole about half way back on the top of the lure, and then sealing it with bondo.
I mix the bondo, spread a thin layer in the middle of a piece of blue painters masking tape, and then put it over the hole, pulling the tape down tight, so the bondo is forced into the hole in a thin, even layer.
The bondo on the tape is about 1/8" thick.
Once the bondo sets, I remove the tape, scrape the excess bondo off the bait around the hole, and sand the bondo over the hole down to flush.
Then I wipe the bait down with acetone, to clean of any bondo residue and to clear up any sanding scratches, and then I put some runny super glue onto the bondo to strengthen it. The glue is actually absorbed by the bondo, so it makes a really good seal and bond to the plastic around the hole.
I hope this helps.