Just remove the three screws on the back to pop the case. Then undo the screw over the card cage to provide space for the card to be installed.
See it now: Sapphire Radeon Pulse RX Vega 56 at Amazon
This is the card I installed, the Sapphire Radeon Pulse RX Vega 56 8GB HBM2 Dual HDMI/DP PCI-E Graphics Card.
The Vega 56 is a big card, which is why the case also needs to be big.
Here's the other side of the Vega 56. That's one powerful board.
Here's where you install the eGPU inside the case.
There's plenty of room for air flow and cooling inside the case, even with the card installed.
Here's the eGPU installed inside the Mercury Helios FX case.