In stead od adding weight to the plane I had the general idea of making a sled type set up maybe slotted so when you move it sort of snaps into place as you slide the weight forward and aft. But instead of lead or outside source why not use your battery pack. Given the weight of your battery you wouldnt have to move it far. Down side is when you want ot change CG you have to land and change it. Which might not be a bad idea anyway.