I use tab for open inventory button, but it's always bothered me that you can't use the same button to close it; instead you have to use Esc or the X button in the corner of the inventory.
Here is some code so you can have a button that opens the inventory, and the same button closes it.
Put it in initPlayerLocal.sqf or similar.
Enjoy
player addEventHandler ["InventoryOpened",{
_this spawn {
waitUntil {
not isNull (findDisplay 602)
};
INVCLOSED = (findDisplay 602) displayAddEventHandler ["keydown", {
if ((_this select 1) in (actionKeys "gear")) then {
(findDisplay 602) displayRemoveEventHandler ["keydown",INVCLOSED];
closeDialog 0
}
}]
}
}]