The shape will determine If it needs to be multiple pieces. I'm guessing it will because the ends wrap around. You reinforce the mold while its on the original piece so it retains the correct shape. You reinforce it with just about anything - 2 x 4 lumber works great. Where pieces of the mold have to attach to make a full mold you make your reinforcements so they bolt together and keep the mold aligned. A multiple part mold is made in sections. You can use plasticine to form a "dam" to make the flanges. Make the first large section and pop it off. Clean up the flange and put it back on the bumper and glass on the reinforcements. Make sure the flanges have mold release applied then make the next part of the mold. Before popping it off, drill bolt/reference holes in the flange. Hopefully this short explanation gives you a good idea of the process. Note: you cannot use too much mold release. When you think you've applied enough, give it another coat.