Well, I didn't have much rice in the house, and I'm lazy, so I ended up with a 20.8# med-ball. Being out-of shape, it will definitely be challenging to use as RX'd all the time, but maybe that will be for the best.
I decided to use an old, ripped pillow as padding. I tore out about half of the stuffing and wrapped it around the ball before wrapping the whole thing in duct tape. After testing it out with a few wall-ball throws, I'm glad I did this, as I've already dropped it on my face once!
All in all, I'm pretty pleased with the result. It feels solid and it will definitely work as a useful addition to my home gym. Time will tell as far as how durable it will be. My only minor concern is whether it will get slick with sweat over an extended workout. Maybe a second layer of textured tape will help with that.