Yeah I've got a handful of chrome wire-grill in the garage cooler collection, the problem with them is I won't be able to use rubber anti-vibration mount for the fan anymore. Will figure that out once I can get some time to mod the case.Perhaps wire grill for the (yet to be modded vent) rear fan? A black one. Fleabay link.
Suggest rotating the heatsinks 90 degrees to push the exhaust up. Yes, that sounds counter intuitive, but you avoid the downstream CPU getting the very short end of the stick. You will see the CPU temps even out.
For rotating the heatsinks, that is definitely worth a try. Currently the GPU lives at the bottom slot so there is large amount of empty space above it for airflow so it might actually work.