Still using the NZXT H510?
It's a negative pressure focused case, but has a weakness in that the higher total system power consumption gets, the harder managing thermals gets.
Side + bottom vent and the unused PCIe slots are the intakes. [If you install front intake fans, the negative pressure system is actually lost.
Gamers' Nexus and
Greg Salazar have videos on this.]
Top and rear 120mm fan spaces are the exhausts.
There's only so much air that can come in and go out of this case. Negative pressure systems can work well, but that one makes things a bit tight.
I recall some sites(can't remember the names anymore) did sell DIY front mesh panels... they might've stopped since the Flow series released.