Kt'hyla's right about people taking the wrong approach on building TMP nacelles. The engines are mostly cylinders and other distorted shapes. The only box on the whole design is the forward section and even it has rounded edges.
I don't know how others do it exactly, but I built mine in sections (front, back, and whatever's left) and then assembled them all into one model for the final product. They're probably inaccurate in a few minor areas (nobody can get proportions perfectly right) but they are close.
I did drop a few separately released engine models on my site for kitbash purposes so people wouldn't have to download my 3mb ships just for the nacelles. At any rate, counting Kt'hyla's and Khaliban's, that's 3 good alternatives to pick through.