I did a search on modifying these lips but only found a couple of threads talking about doing it with some photos of those that had done it. I saw a few people had just cut the sides off and left it at that, which looks ok from a distance. Others had the over hang where the lip extends past the end of the bumper. The steps below show how you can fix all of those problems and make the lip look like it originally came with a straight edge along the top.
Ive had this lip on my car for a couple of years and decided to take the oppotunity over the weekend while my car was off the road to modify the lip and remove the side pieces. I took photos through most of the process so you can see how its done. I didnt stop to take many photos between finishing the glassing and finishing the sanding but applying the body filler and sanding it over and over is not rocket science.
NOTE: This will only work for the fibreglass versions of this lip.
Its not complicated, it will keep you busy for a full weekend and it will cost you under $50 (excluding painting) for the fibreglass, and body filler that you need.
Big thanks to my mate Gary who put up with a substandard dust mask and helped me with alot of the sanding. That brings me to an important point. This will create alot of dust. Get yourself a good mask! I recommend a 3M mask with interchangeable filters for around $100.
Using tape mark the sections you want to remove. You want to come down a few millimetres lower than the very top edge of the lip. This is so you can create a curved edge. You should be cutting in line with the top of the vertical edge before it curves over at the top.
this is now the point of no return. Repeat on the other side.
If you have a look at the bottom edge of your bumper to where the sides of the lip will attach you will notice that they bend upwards. To have a nice snug fit its best to mould the new section over this part of the bumper. To do this you have to attach the lip exactly where you want it to sit.
This may freak you out a little but you have to drill a few holes in the lip in order to get it mounted up to the bumper square. Its easy to get rid of these holes later. I used various pieces of mdf screwed in to hold it in place.
Fibreglassing... the itchy part. If you do not know how to fibreglass read my "How to Fibreglass" thread here. Now for this first part you dont need to go crazy. It doesnt need to be strong enough to attach the lip to the bumper. All we are doing is enough to get the shape and the angle needed. Due to the block attached in the middle to hold the lip in place fibreglassed a couple of pieces about 2-3 inches wide on each side.
TIP: Use tape to mask off areas that you dont want the fibreglass to stick to i.e. the bumper.
Once its dry remove the lip and pull off all of the tape. You should be left with something that looks like this:
Using a piece of cardboard cut out a piece the shape of the lip and tape it to the top. This will allow you to fibreglass the middle where the block was and still retain the correct shape/angle. This time put a few layers on to get the desired strength.
TIP: Cover the cardboard in tape first so it pulls off easily.
Once its dry pull off the tape and carboard and it should look like this:
Using an airsaw trim off the excess. And check that the lip still fits snug on the bumper. Check that the new section is thick enough. It should be 2-3mm thick. If its too thin add a couple more layers before continuing.