A button box can be within arm’s reach and still get in the way of your view. Sit in the rig and load the cockpit view before marking holes or tightening mounts. The screen will tell you more about the layout than standing beside the frame.
Give the driving controls first pick
Keep the controls you use while moving within an easy reach. If a button belongs on the wheel and you have an available input there, map it before adding another box. Hold the steering wheel with the base off and turn through the range you use. Check that your knuckles and paddles clear the proposed mount.
With the sim on screen, check the section of the display where you see the next corner, mirrors, flags, and any information you routinely glance at. Hold a cardboard cutout the size of the device where you plan to mount it. Try the same check with the seat in a second position if you share the rig.
Set a place for the rest
A keyboard can live to the side if you use it mostly in menus. Put a phone or tablet where you can check it without it hiding the screen. Leave room to move a microphone boom and remove anything that has to come off the mount regularly. The rig accessories collection has examples of holders you can plan around.
Run one practice session with the layout before tidying the cables. Any time you reach past something or look around a device, mark that position for a move.