On a large and medium, the grill dome extender works very well. It is over 5" so when it site on a grid on the fire ring, it is between 1" and 2" above the felt line. Not sure if they make one for the XL.
And further to what @lousubcap says, a single 8# pork loin will cook in the same time as one 4# loin, It is the cross section of the meat that determines the cook time, that is how far the heat has to penetrate to reach the centre. Typically a pork loin might be 4" to 5" in diameter regardless of wether it is a 4# or an 8#. The 8# is longer, but not thicker. Cooking times will be the same, Same goes for prime rib.