My LFS had this kid filling in one day. I decided to buy a frag and his packaging method seemed odd for a minute....but I quickly realized it was godlike. He got a square of styrofoam and rubberbanded the plug/rock that the frag was on to the styrofoam. Filled the bag with water, then plopped the frag in the bag upside down. The foam obviously floats, which keeps the frag always under water. Also, if the square of styrofoam you use is larger enough, it keeps the frag from brushing against the side of the bag too. Blow the bag up tight and secure it shut. Done and done.