Take a DSMB, Cut it down to a meager/useful size, and you have a great little system....
Between AquaSeal, Stitching, etc., I'd bet you can build a small float out of a DSMB....
But, why is this better than a small soda bottle? It's something I would deploy at the start and take up at the end, so it's not like I'd be swimming around during the dive with a soda bottle clipped to me.