I'll preface this by saying I may be in the wrong place, so if the Mods have to move it, feel free.

Now that I'm close to finishing the new build, I'm looking to see if there's some way to put a fan behind the 3090 FTW card, to help keep the back side cool.

It will be a vertical mount, in Thermaltake's AH-T600 case. The case itself is supposed to have good airflow, being an open design, but I thought I'd explore the possibilities for cooling the card better, especially since the vertical mount is directly over the PSU.  I've seen some brackets that allow you to put a fan over the top of a card, but I was thinking of something that would either blow parallel to the card, or directly at the back of the card.

I've got the Z590 FTW board and an AIO cooler, so the CPU/cooler shouldn't be in the way of anything since it sits up high(ish). I was thinking of trying to sneak a 120mm fan in behind the card, but might have to opt for something a bit smaller.

There may not be such a beast, to be honest. There's a good chance I may have to figure out how to ghetto fab something myself, but thought I'd toss the question out there and see what I might find.

Anyone have an idea of what I might be able to use or adapt?

I will just say, I put that Bykski active 3090 backplate on my step-sons computer the day before yesterday.
$50 active backplate.
Dropped the memory temps down to the high 50's.
While mining and a hefty ram overclock, I saw the RAM temps mainly stay around 56-58, occasionally 60 but later to fall off back into the high 50's.
 
I for real was thinking uhoh, I hope this thing doesn't blow his card up.
 
I remmoved his old backplate, removed the front heatink sinks, the new plate comes with a boatload of screws in various sizes. so the screws go through the active plate, through the card into the front plate/heatsink and sandwhiches the card between, so there was practically zero bow at all.
 
Now he put the cheap blue thermal pads that come with jsut about anything you buy these days, with some decent pads, I would guess a couple 3-4C drop more, putting this right around 55C RAM full beans.

When I had a fan cooling my 3090 backplate, I just put a 120mm fan on the backplate with a piece of plastic under one side to keep the fan up at an angle so that air would flow out from under the fan.  I thought about putting rubber feet on the fan to raise it up, but never did.

Nozler
Something like this modded a bit for vertical?  https://www.newegg.com/p/2T3-003J-00001

 
Solid find.  I'll probably spend a few hours looking for something like this all day Saturday now.

Need to find some brackets so you can butt the fans together and get a single slot support bracket.  Looks like you could rob the parts from 3-fans/lot ADDA AD0912UX-A7BGL DC 12V 0.50A VGA Graphics card cooling companion PCI fan - Newegg.com.  Then you could make your own (if you wanted to add LEDs etc).  Wondering if you could do something simple like the Lian Li SL120s. Guess the problem would be the pci-e port support, but might not be so bad if you have a vertical installation. 
 
Amazon.com: 80-90mm Dual Fan Rack Mount PCI Slot Cover Bracket Video Card Cooling : Electronics
 
Maybe this?