I recently had the occasion to try and figure out why all the downspouts around a home were completely blocked. When I finally decided to take them completely apart, I discovered that the workman who installed them had a really nifty way of making the colour match between the aluminium siding on the top and the brick on the bottom fit perfectly -- he simply collapsed the top one and jammed it into the bottom one until the colour transition was at the right place. One of them was jammed in 18 inches, rather than cutting it off to a reasonable length. In fact this was his very quick technique for all the fittings all around the house. I took a photo of the results -- two feet of compost starting right at the narrow spot. He obviously did not own a pair of crimpers.
The first photo shows a very unique set of pliers -- three blades on one side and two on the other. All they do in life is crimp up sheet metal to create one end that fits snugly with a good transition into the next piece -- just like on the manufactured edges. For any air ducts or downspouts you should have a pair of crimpers and the work is so easy.
If you don't have crimpers, at least avoid the squash method. Take a pair of needle nose pliers and by a clamp and twist action, you can create something close to a proper crimped edge, one fold at a time. It is not as smooth and even as with a crimper, but it does work and you won't be blocking all the leaves at the joints.