Is there any material that is harder than the bricks but lighter than PVC?

Density of PVC is 1.3 to 1.5 times water. Brick is 1.7 to 2.1. Hardness of brick is uncertain, no matter what it is made of.

That is a very odd comparison. What you need to do is figure out what you want and then it's fairly easy to find it. For instance you can get glass bricks that are very hard and will float, but they are easily broken. You can have steel boxes made, and they can be very durable.