My solution for this was to take a scrap of 2x4 and use a square to mark a line down the length along the top of it. Then I flipped the board over and marked the line on the bottom (using the same side as a reference to make sure they were equally far from the edge). Then I measured from the end every inch or so on the line on both the top and bottom (so I'd have multiple shots at it). Now I have lines set up on the top and bottom that should be directly overlapped, so I freehand drilled at each of the marks until I got one that hit both marks perfectly, and then I used that hole as a guide to drill through my marks on the body. That didn't drill all the way through, but the new holes in the body were deep enough to keep it straight the rest of the way through. It turned out pretty well. I was using rivets (with the pins cut off) for my ferrules, so I used a router to route a small recess on the back side of the body, which cleaned up any scraggly holes from drilling through.