Trying to protect a genset from under-vehicle winter spray is a fool's errand. Any kind of covering is likely to trap and hold salt, making things worse. @JohnNM has the right idea. Spray will protective products, and wash down whenever possible,
Here's a relevant story:
The DeLorean, although having a totally rust-proof SS body, has a mild steel frame, which was thus rust prone. Aware of the problem, they coated the frames with epoxy. This was great, except that after a few years, the epoxy formed cracks. The cracks collected and held salt spray, often causing severe rusting. They likely would have been better off without the epoxy.