I too, have constructed my own flares, and though I don't think I have any pics conveniently on hand, I can describe the process somewhat simply for you.
You start by deciding what size and shape you want the throat and mouth of your flare to be. Typically, the mouth is obviously going to be larger than the throat. Once you've made that determination, you can simply cut the openings into your pieces of MDF, and space them however far apart you want, to make the frame.
Next you will take some polyester fleece and staple it smoothly all around the inner edge of the throat, and then stretch through the mouth, pulling the shape tight, and staple it in place. Now simply coat in polyester resin, let it cure, and voila, custom flare. I did, once upon a time, make an enclosure with one flare for the woofer, and two smaller flares for the 2 ports. I'll have to see if I can find any documentation for it.
Being fiberglass, it will have a rather rough finish, on which I recommend sanding down the high spots, and then spraying with a high-fill polyester body filler (basically spray-on bondo, available at a paint and body shop supply house). It's much easier to apply evenly than regular fillers, uses the same MEKP catalyst as the resin, and sands very easily. If you catch it midway through the curing process, when it's somewhat rubbery, and hit it with a 60 or 80 grit paper, the high spots will pretty much just fall off with virtually no effort. I absolutely love the stuff.