I think the cost of fabrication of that will shock you unless you find a real friend with the skills. I bought a stainless "fit most anything" cap for less that $60 at Lowes.
Thanks for the info JohnAL, I know fab shops ain't cheap. They got bills too. I'm a metal worker myself (just not at my house) and thought about screens or expanded metal and all, but I just don't know if the louvers are there directionally for wind flow, turbulence or what. I do like OEM stuff but may need to look to Lowes like you said.
I had this place fix up my chimney. House has been owned for about 7 years and never burned in it. They were pricey but we needed a ton of repairs since our unit was so old (house from the 70's.) For the price the value was there for us. I burned in it last night and every chance I get. https://pensacola.arcchimney.com/
I found them in one of the mail flyers. They advertise a 10% off coupon. And they honored it just for mentioning it.
The fireplace shop I went to is just north of Yamotos resturant on New Warrington rd. Really nice people he recommended a guy that does chimney repairs. Fireplace Concepts 625 New Warrington Rd.