Hey Tony,
It was trial and lots of error!!!!!
I started with metal... Welds were too bulky and the vents were way heavy... I then tried aluminum, it looked bad... Welds were too bulky also...
So..... I actually made these vents using 1/8" ABS plastic sheeting... I cut all the slats equal width then glued them at an angle into a formed ABS hoop of the same size as the hood opening I cut in the hood... I then added/glued on the "flange"...
You'll also notice, I notched the slats and glued a 1/4" deep vertical slat dead center of the horizontal slats to stabilize any flex/flutter of the blades I may have had at hwy speeds as my vents are "active" for my rad cooling air flow thru.
Not sure exactly how I will mount them, may use two sided 3M tape or rivet... The tape would be less intrusive and reduce rust issues... meh.....
A 4x8 sheet ran me about 65 bucks... I bought mine local, but below is what I used...
http://www.eplastics.com/Plastic/abs-plastic-textured-general-purpose-sheet/ABSBLK0-187HC48X48
I still have enough for my interior dash plates along with door panels.
So.... The fun part... Putting it all together... I tried multiple types of glue... From cyanoacrylate to epoxy to gorilla glue to heat welding... Best out of all??? Small scraps of the ABS mixed with acetone to a consistency of Elmer's glue... Mostly the "sawdust" from cutting the ABS slats mixed with acetone...
The acetone/ABS mixture softens up the ABS for bonding and fills gaps nicely...... Once the acetone evaps, the ABS solidifies... strong as crap! Pretty quick setup too.
I have dropped these from an upper shelf 8ft high in my garage... They held together no problem... I left them out in the Texas heat all day, no warpage no delamination...
Pretty cool huh... Almost forgot, AFTER I was satisfied with the vent design and assembly, I scuffed them down and shot a light coat of Duplicolor bed liner in a spray can... This stuff is amazing... Have in on my bug now for almost 5 years, still looks brand new...
Hope this helps...