The easiest thing to do is buy the aftermarket cowl. I usually fiberglass the entire inside of the cowl using slow-drying epoxy - not ca. The seams can be filled with anything that will stick to the plastic and has about the same hardness. Auto-body filler is the best choice, but you can also use medium CA to fill the seam. A lot of plastic modelers now use medium CA as a seam filler between fuselage halves or wing halves and it works very well.