Fastacraft Facebook page

In lieu of any pictures that might give some pointers, you could try using closed-cell foam to create the shape that you need

The EC airbox tutorial video sets out the basics of creating shapes from scratch, using sections of thin sheet foam to make shape profiles with and these can then be in-filled with cut blocks of foam. It's easy to sand the foam back to the shape that you need and the thin profile sections will help you to keep the shape uniform and consistent. I've used this method to create patterns from scratch before. The foam sections and infill blocks can be bonded to your existing part at the outset using hot-melt glue, for example

Car body filler can be used to further smooth the pattern over, once you're happy with the foam shape and a thin application of woven glass can give the new pattern some strength, before final shaping/filling and blending-in with your bumper. Once happy with the shape, you can move onto painting and prep for moulding. It takes a bit of patience, but it's a relatively straightforward way of making new patterns.

It's hard to find a picture for the bottom shape of the bumper. I have the plug created, but it took a very long time to sand, filler, sand, filler..., and at the end the final shape is still not 100%. But it may be just I don't have enough experience to start with. I used insulation foam to created the flat area, then used small pieces foam to create the curve portions that lead to another level which is half flat, half curve. After the general shape is done, I fibreglass it, then primer again and sand again.  The process took too much time, and final result is not 100% satisfied. I would rate it 90% at the end after painting and polishing.

I watched the air box video after everything is 50% done. But my plug is 6 feet long like a 'wide-U' shape bumper lip, but not sure how it will turn out if i used the air box technique earlier.

I like the 3D CAD software and print out the profile of the shape, but I don't have the dimension of the bumper. But I love the print out then cut the foam base on the printed profile, it gives much more accurate shape. 

Thanks for the suggestion. I will give it another try on my next project.