The basis of this technique is something that I picked up from a friend for making compound angle cuts. However, I decided to use it to see if it might also be able to simplify cutting some types of 2D tubes as well. A diagonal tube between parallel planes in a rectangular opening is relatively easy to math it out. But it adds another layer of complexity when the opening is not rectangular, nor symmetric for that matter, and the tube is not between parallel planes. Thus I figured this would a perfect time to practice an alternative technique.
First I spent some time figuring out exactly where I want the tubes to go. These tubes also happen to be going beneath the transmission. So I had to make sure to avoid the lowest parts of the transmission, try to leave at least some emergency access to the drain and fill plugs, and adhere to my personal guidelines of avoiding overlapping weld joints and angles tighter than 45 degrees. The latter of which I forgot about last weekend until after the tubes were cut, sending me back to the drawing board...Literally. No fancy CAD programs here. Just good old fashioned graph paper, a pencil, and a ruler.
The process then starts laying painters tape across the span where the tube will go. Measure out the desired corner points based on the design, marking them with a Sharpie. After that, use a large enough straight edged device and play connect the dots.
Since the Sharpie can be seen through the tape, I find it easiest to lay the tape sticky side up on the table and align the tube on top of it. Then flip it over to clearly see where the cuts will go.
Now my band saw only angles in one direction. To cut angles the other direction, you simply flip the part over. With both cuts going opposite directions, only one of them can be cut with the tape side up. Align the cut line on the tape with the cutting blade, lock it in place, and let it rip.
With the tape having been cut as well, it now becomes the transfer pattern. To make it stick to the other side of the tube, place a couple strips of tape sticky side up on the table, lay the transfer pattern tape sticky side down on top of them, and align the tube on top of that matching up the previously cut ends. Now wrap the sticky side up tape strips around the tube to hold the transfer pattern in place. Roll the tube over to see where the cut needs to go, and using a center punch or scribe, mark the desired cut line in the tube.
At this point the tape can be removed, revealing the line to cut.
I knew from the start that this method might provide slightly less accurate cut positions and angles than I get from my hand calculations, but it proved to be quite a bit more accurate than I had anticipated. I was impressed with how easy this technique was to get a feel for, as well as how little 'massaging' will be require to finalize the tube fitment. Just remember that it's a LOT harder to cut material back onto the tube, than it is to cut more material off.