Check out this poster on Reddit who created custom plexiglass top panels that can mount 120 and/or 140mm fans. (Here's hoping Noctua comes out with 140mm slim fans in 2021)
He even
included .DFX files so you can make them yourself.
After spending time getting my DA2 air cooling 'well', I decided these kinds of things are exactly what the case needs.
Clearly I'm not alone in thinking that... cool to see these panels.
Rigid mounted elements using the horizontal rail grub screw holes to keep in place, with fan attachments directly to that rigid mounting, and shave out as much wasted thickness as possible.
Make them in aluminium or steel and you could get them pretty thin and gain precious mm in space, especially if you push the panel right out to sit flush with the edge of the case rails (just dropping down right at the edge to pick up the grub screw zone)
Really though the DA2 just needs to be 10mm taller above the mobo... that way, no matter what you do at the bottom end of the case under the gfx, you have a 25mm gap at the top so you can fit decent 140mm x 25mm fans at the top.
As soon as you have that pillar of air moving slowly and broadly across the interior you're somewhat guaranteed decent temps and noise levels.