It's almost fully covered... too much heat will stay inside.
Force air cooling. If you force cold air IN unless you trapped a blackhole inside the hot air will be forced out. Now you are right if you just slapped together some shelves and cards that would be bad. It would require some "ducting".
Don't laugh at my ASCII "art".
hot air out pulled out
^
| |------------------------|
| <- bank of GPUs <- |
| |----------------------| |
| <- bank of GPUs <- |
|------------------------| |
| |
^ (cold air forced in)
All the ^ are fans indicating direction of airflow.
Only two banks shown but more could be used.
Hooking all the fans to a fan controller with some temperature probes in each bank could be used to optimize cooling. Upper bank would need higher airflow because air is lazy.