[Mack_JB]

For the HGEV, carve the single barrel off the metal mini, and replace it with two pieces of wire or styrene rod, one mounted on either side of the turret.

For the MGEV, I have looked at merely gluing the missile rack from a plastic MSL tank onto the top of a stock GEV, perhaps only cutting the round mounting pad off the rack, or shortening the overall height of the rack as well.