DIY depends on your or, possibly your friends', skill level and tool access.
Personally, I think a DIY solution is very doable. I was actually heading down that path myself 10ish years ago, until I stumbled upon a deal on on eBay for one of the aftermarket designs. I don't see such deals available today. No, it doesn't fold up like the factory design, but it's stronger and I would never subject anyone over age 12 to riding in the backseat, anyway. And since my youngest is now 22, that's not likely to happen.
I haven't worked with 3D printing, but would imagine the size of the deflector frame would be a challenge for most consumer/hobbyist type printers. Then there's the strength needed for edge supporting that much air volume at 70+ mph.
If I were doing it, I'd probably start by templating the opening over the rear seat. Cut a perimeter frame out of 3/4" plywood using maybe a 1" wide border. You could consider leaving a coupe of stiffening ribs across the middle. Use your favorite no-see-um window screen to cover the panel. Make a similar frame to hinge up behind the front seat headrests. When I was considering DIY, seems like I found some fence or possibly trailer gate spring loaded latches that were suitable for the mounting points in the rear side panel. Attach those to your frame and Bob's your uncle.