I use acetone to join plastic cowl pieces. just make sure you use a thin strip of plastic on the inside over the joint. make the strip 1/4 to 1/3 inch wide. use the excess from trimming the cowl, or buy some plastic strips. take one cowl half and hold the strip on the inside with half of it extending past the edge of the cowl on the side where it joins the other half. let some acetone (a few drops) run down the joint. acetone effectively "welds" the plastic pieces together. now you have a cowl half with a lip on it to join to the other half. follow the same procedure to put the halves together. if you sand some scrap plastic and mix the dust with acetone you can create a filler that really bonds to the cowl. hopefully this gives you some ideas.

-Aaron