I didn't want to add this to the box flares discussion -- just wanted to add another way to add flares to a car.
A little background -- I needed fender flares added to my TVR to cover the 315's and 17 x 11 inch rims. It was simply not possible to narrow the suspension enough (C4 IRS) to get suspension/wheels/tires inside the bodywork of a narrow British car. So I started looking around for a set of flares that might work or could be adapted with little effort. FYI....there is no off the shelf part for a TVR flare 
In my case, the answer was curiously enough, Miata! Moss Motors carries parts from AWR. The Flares for a late model Miata were a very close fit. The fronts, swapped sides, were a good match for the rear, and vice-versa for flares for the front. The only issue seems to be that the Miata might have some more curve to the body whereas the TVR is a little slab-sided. The flares stuck out from the body at the leading edge -- they also overlapped the door a bunch.
I got them to fit the best I could -- and they did with very little trimming. I cleco'd them to the body and epoxied them to the body at the rear and where they touched the body under the door. I then notched out the flares so the doors would open/close. I also spent a lot of time making sure the flares were symmetrical side to side, and that they didn't interfere with the wheel/tire under full suspension compression.
Now the fun began. I had gaps where the flare just wouldn't meet the body at the leading edge behind the doors, as well as at the peak of the flare. TAP Plastics makes this cool stuff called X-30 expanding foam. I love this stuff!!! Mix equal parts, wait less than a minute and the liquids expand to 30~40 times their volume in about 3 minutes. Foam is fully cured and ready for shaping in less than 30 mins! I shaped the foam with nothing more than a japanese pull saw to rough out the shape, then sanded to shape with a sanding block and 1 sheet of 60 grit sandpaper.
Pics show a before and after. I'm just waiting for the weather to warm up a bit before I start to glass over the foam.
I think this is a slighty quicker route than trying to mold the entire flare from scratch -- and I started out with a pair of flares that were symmetrical -- something I was not so sure I could pull off from scratch. I did find out that the fine British handiwork of the 60's and 70's did induce some variation side to side on the car.
.......yeah, the radiator intake isn't exactly symmetrical yet. Noticed that after I cut the shape out of the foam. Currently working on it -- using a little old bondo that has thickened up a bit to build up the shape and make it the same side to side. Once all that is complete I glass everything in and then remove any foam/bondo/temp 'glass from the backside and lay in a couple reinforcing layers of new 'glass/epoxy. Just learning as I go......
