Depending on how much weight you were trying to keep in place, and on the configuration of the volume of the space in which you wanted to place the balast --- I'm assuming you're trying to trim the boat with different balast configurations, and this is temporary --- there may be another option.  You may be able to hold balast in place by locating it where you want it and then insert and inflate a relatively strong beach ball inside the area and above and around the weight so that the balast is held in place. Not good for a long test, because the ball will leak over time --- or heat up and explode! --- but, in the short run, it will hold a fairly substantial weight in place for trimming.

For Gabriel: I assume you *don't* want a picture of this!

 I do actually, but may try it myself and take a photo.