Firstly, kudos to the OP for thinking about this issue. Noise disturbances are an easy way to alienate neighbors (or get into trouble with noise pollution regs).
I can add styrofoam to the cheap end DIY options. Ask around for people buying white goods to save you all that good (free) white stuff.
I guess this may be a better tip for future projects as it will take time to collect enough, unless there is a way to buy it directly from a supplier.