The key to spinning off boxes is the grip. Gives you good balance on the box and allows you to push off to get a good spin. If you want to spin fs out then grip with your front foot, bs grip with your back foot.
Gripping helps with switchups too, only fs though at least for me. Just because a fs switchup is like a fs spin out. I don't grip with bs though, that was the first switchup I learned and I think it's the easiest. What I did was over rotate my head, like when I land on the box I keep turning my head and shoulders. Then I just hop up real quick and switch up. It's easier than you think, it's all about momentum, balance, confidence, and the wind up. Just gotta throw it one day and you'll see its not that hard. Also they may be scarier but I think switchups are easiest on a kink rail.