Ben Pearson solved this problem with their biogrip bows. The grip is angled at 20 degrees which keeps your forearm out of the path of the string. It's the most comfortable way to hold a bow, but it's only available on Ben Pearson bows. Take a look at the Pearson Pride, for example...



Or you could just get an STS.

I like to shoot with my bow arm straight. I'm more stable that way, and I get more speed due to the increased draw length. I use an STS and it eliminates all forearm slap, even with a heavy coat.