I am in the process of doing a major refresh of my home's exterior. I need some help with one area that has me stumped. I have round attic vents that were originally installed with the exterior siding. I replaced the siding with Hardy shingles. Because of the layering of the shingles, the vent is no longer flush. It is too late to replace the vents so I decided to put a frame around them. What kind of lumber product would be suitable to build the frame? Outdoor plywood would work but is there a better alternative?