Judging from that pic, means that you need to make your own guides for it from plastic, or even make it travel out of the back and then over the case and out.
By trying to force the air to change directions by 180 degrees, you would be adding a ton of airflow resistance to the "inside the case" card exhaust. That resistance would cause a (relative) high pressure zone on that side of the card. The high pressure zone would cause the majority of the airflow to go out through the "outside the case" exhaust and thus the majority of the air to flow over the "outside the case" GPU and a significantly smaller amount to flow over the "inside the case" GPU.
This is nearly the equivalent of pointing an inflow fan (which is very likely to exist right in that area of the case......) directly into the "inside the case" card exhaust. They do nothing but fight eachother and make everything worse in total.
In a worst case scenario, this could even cause a resonating normalflow/backflow which would cause hot air to stagnate on top of the "inside the case" GPU.
Such a cover device would cause MAJOR problems for the card.
The only proper solution for this extremely significant airflow design is to change the lower forward intake fan to instead be an exhaust. You will then need to find another way to flow cold air intake into the case. Both to the intake of the GPU and the CPU... Doing this in many cases would not be a trivial task.