I totally feel you on the spazzims. I broke my tib/fib last week, and they decided not to cast it because they put a rod in it and they said the rod would hold it and casting it would promote atrophy. It's definitely more comfortable not having a cast but the spazzims kill. Just do whatever you can to relax the muscles. I've been propping it up with pillows and shit, but whenever I have to get up it kills.
Another thing I reccomend is to make sure to keep you're elbow moving, it'll cut down the physical therapy later. I broke my unla and elbow a couple years ago and had to have it casted and when the cast came off I couldn't move my elbow at all, which was frustrating.