Hey newlite:
No doubt your bamboo bushes will hold some crappie if placed at the right depth and a good location. I don't think you'll have any trouble getting them to stand up because the bamboo is buoyant and the block will go down first.
For about the same expense and just a little more time and trouble you could use a 5-gallon paint, drywall or hydraulic oil, etc. bucket and make a much wider and taller condo that should hold more fish for longer periods of time.