The weight of hydroton brand horticultural clay pebbles also known as "Leca", Lightweight Expanded Clay Aggregate weighs 23.67 pounds per cubic foot. 47.34 Pounds = 21.4731 Kgs. This should be roughly half the weight of typical round rock gravel of similar size.
If there's a landscape supply near you, you could ask them if they have rock bark (or another lightweight large-size gravel). It's like 10% of the cost of clay balls.
Not that you need very much :)
go to google and type "liters in 2 cubic feet" The answer is 56 liters. I usually buy 50 liter bags, so 1 bag would do it.