I used 4pcf mineral wool and aluminum angles for my cloud. It hangs on chains from eye hooks. Very light, but the aluminum wasn't cheap. Look around for something similar, you might be able to find the same thing for less.
Rockwool / Fibreglass will perfom better at lower freqeuncies than foam. Foam kicks in about 250hz so ok for the higher / mid frequncies
Here's some testing results we did in our own lab comparing the two - Comparing Foam to GIK 244 Bass Traps