This organizer is sized to fit inside the mars box and is 50mm high (it is approximately 70mm wide depending on how you measure it). This is as high as you may make something that will have both game boards on top. (That is, if you have the base game board and the Hellas & Elysium board in the box, you can place this on top and it will meet the top of the game box's lip).
The lid does not snap on but assumes the game box will hold things into place. (Time will tell if that's reasonable. I rubber band would be a good way to hold it together (along the side of the organizer which would apply frictional pressure while not increasing the height of the organizer).
This was designed in Fusion360.