The cheaper ones won't hardly last a couple of years without becoming brittle and crumbling, if they are exposed to the elements year round and not kept up or covered. I had to replace several on my brother in laws unit for that reason. Could poke my finger through them. The Fantastic Fan covers are way thicker and better made than the Heng, Ventline or other cheaper ones. Either way, we just always put the MaxxAir II covers on all vents right when we buy one, and have never suffered casualty on one. As Wiscampsin said, probably best to take one with you and match it up. Or, look at the brand and do some searching. 14x14 is the standard.
John and Rebecca Dickson
Emma-7 / Little John-5 / Iva-1
Full Timing Again, Rev B
2013 Ford F-350 Lariat CC LB PSD
2015 SOB TT - With OC's Awning Poles (#8)
At least in Heaven, RVs will be perfect, and I won't have to keep fixing them.