Shoot, weighting it down doesn't work very well anyway, as there is so much CO2, it comes to the surface anyway, for the first week. I guess a baseball-sized glass ball would do the trick, though.
I take advantage of this fact, when I want to ferment to dry in the primary bucket and I want to leave the grape pack in the wine a little longer.
You don't want the grape pack to float unless you are able to punch it down a couple of times a day. A floating grape pack can start molding. However, once I snap the lid down and add an air lock, I don't want to be opening the fermentor regularly to punch the pack down.
So, I put several pounds of marbles in the bag, which still lets it float for about a week. By the time I get ready to snap the lid down and add an air lock, the CO2 has diminished enough that the grape pack bag will finally sink and stay down.
The amount of weight needed is a trial and error scenario, so sometimes I have to tie on another bag of marbles to the grape bag just before I seal the fermentor. You surely don't want to open the fermentor after a week and find the grape pack floating and smelling musty.