Bondo is not for filling holes. Its for leveling and filling small imperfections, like dings or low spots. An actual hole is gonna need to be covered with something such as fiberglass.
However, I've never repaired a plastic bumper so I'm not even really sure how well fiberglass will do. Anyone with more experience than me is more than welcome to chime in.
The two part epoxy does a great job on bumper and plastic, We use it all the time to weld ABS and repair Dash parts. Maybe use a patch peice from a parts cars and use the epoxy to put it in place, Fill if from the front and rear and then sand the front smooth.
Go to a good auto paint store. There are 2-part repair products specifically made for urethane, which is what our fascias are made of. I used Duramax 4030 Flexible Bumper Cover Repair to remove the "bump strips" from my rear bumper (my front is fiberglass, so I used glass there). It's been 12 years now in climates from Texas to Michigan and it still looks good. It's not cheap, but it's really the only way to repair a urethane part, since the original material is so flexible and resistant to adhesion that nothing else will be a permanent fix.