Hello EnduroTEX and welcome along.
Powder coaters have a habit of doing stuff like this. It happened to me last year with a sump shield.
He swore black was white he had not got it. I got in an argument with him because of this and then produced a picture on my phone i took of my stuff at his shop. Showed it to him and he said "Oh i'd better have a look in the machine". Sure enough it was buried at the bottom of the accumulated grit at the bottom of the machine.
Drop them a bill for the part.
Not your fault they lost it.
I'm sick of folk getting the f##k off tablet from people doing paid work for them.
Stuffs dear enough without ass holes making it dearer.
Seriously EnduroTEX get em told !
Regards TED

The cover would be very simple to make. It is just a tringle.
If you took your air box to a HVAC shop that makes ductwork, they could cut you out a triangle, bend over the edges (if you want to go that far) and send you on your way in a jiffy.
They might even do it for free.
Drill three holes, paint it black.... install and enjoy.

Welcome, post some photos.
I'm on several other forums, this is by far the most friendly, along with a wealth of experience, knowledge and humor.
Its kind of like sitting around the campfire, you know, the kumbaya thing.

Not a bad solution in a pinch but the cover is a little more intricate than a flat piece of sheet metal, if you look at the design of the cover there is a indented triangle in the center of the metal cover that the end of the filter aligns itself in and to keep a snug fit from end to end of the filter. There is also a little threaded tab on the cover which allows the plastic cosmetic side cover to be screwed on to the filter housing cover, the plastic side cover is strictly cosmetic and to conceal some wiring.

It is also one of those parts someone probably has 3-4 of them lying around or on junk bikes, the ones on eBay where they want crazy money are hoping to find buyers in a predicament as you are in, hopefully someone will chime in ad send you one for the cost of postage and a beer.