I know pattern drafting is it's own beast, one that I don't really know - but what you could do is start with a pattern you can find and sew it up with a basting stitch in leftover fabric/thrift store find/clearance muslin as a mock up leaving off all the nasty details like pockets and such. That way you could alter even a so-so pattern. The problem is it still takes time. But then again I'm only fair at sewing and I almost always try to make a mock up before a final project if the final will take time or money