One of the easiest ways of adding ballast is to simply use a quick link or three to attach the shock cord to the nosecone.
Do locate the CG as described in the post above and then add quick links until it balances where you want it to. As a "general" rule on most rockets, the CP is just ahead of the fins. If you have sim data that definitively locates the CP, use that and add balance weight as needed to get the CG one body tube diameter (BT-80 = 2.6", etc.) forward of the CP.
Kit (AKA Cranky Kong)
Total Total Impulse as BAR: 7,753.69 Ns (Equivalent to a 51% M motor.)
=| Calirado, Colofornia...what's the diff anymore? |=