If you want to make a fiberglass one this is how to do it.

1. Wrap your bumper in tape to prevent the spray foam from permanently bonding to it.
2. Use spray foam to make the shape of the lip you want.
2. Carve and shape the foam exactly how you want it.
3. Bondo the foam and sand and shape again.
4. Wrap the foam in fiberglass and resin it
5. Sand and paint

This is a simplified version but it gives you the idea.