If you check the sticky at the top of the forum thread listings there are links to some tools. One of them is a CG location calculator. Your middle picture with the sunburst stabilizer will prove just fine for this kludge. Measure it as needed for the calculator and I'm sure you'll end up with the CG in a normal mainstream location. It suggests between 5 and 10% be inputted for the stability margin. If you're a fairly competent flyer after producing all that carnage then do a couple of calcs with 8 and 4 inputed for stability% values. That would be a good max forward and max rearward range to work between until you get a feel for the model.