As Julie said, it's not necessary to keep it down provided you keep it moist and stirring it into the wine is a good idea. If you want to lower the bag, use food grade marbles.
Agree with the above. But if you must weigh it down, a sanitized "rocks" glass or coffee mug will do the trick. I just did that back in December when I had to be gone for a couple days during primary.
The stir spoon I have is the perfect length to angle from one bottom edge to the opposite upper side. Trap the bag on the bottom and use the lid to hold the spoon in place. During primary fermentation you will need a little weight on top of the lid. During primary I still stir twice daily as instructed working the bag fairly aggressively with the spoon trying to get as much goodness as I can from the skins.