Mcmaster carr lists green G10 as suitable for outdoor use and yellow G10 (stronger) as not suitable.
Seems like the most commonly used plastics for outside use are PVC (conduit, electrical boxes, windows, siding, etc) or PE. These would not be nearly as rigid as g10, not sure if that would be a problem or not, or if you can increase thickness. Delrin is a dream to work with, fairly rigid and can be used outdoors in some cases, might want to do more research on UV resistance before buying it.
Could you do an aluminum ring with small standoffs so the coils dont contact the aluminum?