I wouldn't try to make a sport coat without a pattern, but I'm willing to wing it when making a rectangle with a hole for the head.
Many years ago I knew an elderly gentleman who had an unlined hooded cape made from loden cloth. Sometimes I'd take his dogs out for him, and if it was raining, he'd always say, "Wear the loden cape - it gets warmer when it's wet." I never believed him about the 'warmer' part, but that wool cape was indeed comfortable and it shed water perfectly. The price today of a loden cloth garment can be pretty startling, however.