Lots of great input to this problem! TBN is the ultimate team. This thread reminds me of building model rockets when I was a kid. To get the fins on straight, the instructions showed to place the fueslage tube in the little corner of the front of an open drawer, and put a pencil line along the tube length (similar to the first suggestion you got from BCP). then another trick was, cut a strip of paper to length equal to the tube circumference. Now, depending on the number of fins you are applying, typically three but sometimes four, fold the strip of paper to that many equal spaces, or use a scale and divide the paper with marks. Now wrap the paper around the tube and transfer the marks. Back to the drawer to make the marks into straight lines, and glue on the fins!