For the curved aspect of mounting your ac unit, there are a number of companies that make ac shrouds and foam gaskets. With those kits you get additional shims that run down both sides. I just looked up the part and a company called ventmate makes them. I happen to be at work and was able to look into it
Edit - I talked with a couple of journeyman rv mechanics and they will try to use the foam gaskets, they are good for up to an inch gap. Most said they have never seen a unit in which that was nessisary. If the gap is larger they would build a custom box made of 2 by 4 that could be shaped. Then seal it up well with paint and such. Another material they use is the cutting board material. It works well for shims on the exterior like awnings and such. Cuts well witha table saw and is sandable.