Yea, they do move right, and feel good. However, there are a lot of horror stories about water balloons bursting at the worst possible moment. You can use a nylon stocking with bird seed, rice or some such (good for weight and movement) Or, polly fill (pillow stuffing) works well in a stocking. You can also cut a hunk of foam rubber to shape. You can get that at a craft store or out of an old sofa cushion.
You also mentioned this isn't your first rodeo... Do you have forms already? Bring them out! Besides, it sounds like you might like to have a set anyway. It may be time to invest.
A bit of advice on that, some web sites that sell post-mastectomy forms sell one at a time. Make sure you're getting a pair!
