My opinion?!

Dont' use the bag. You want the bulk of the 'added wieght', the lead, to be a part of the plane.

How would you secure it? if it came loose, you could have a potentially big problem!

Epoxy the lead in place (balance the plane on it's nose) and let cure. final balance with the radio and gear as far forward as possible. You might need to add some more afterwards, and that's easily done.