Regular old coarse builder's sand is perfect. They can also use this for grit so that will save in an expense there (hubby will be happy...lol)Oh sands sounding better by the minute! Lol can it be any kind my husbands in construction so I have plenty of builders sand but my neighbour said that's not good for them, he suggested kids play sand but that's quite a bit more money! He has cockiteils btw so not a chicken expert just an opinion so if he is wrong please let me know.
Corrugated plastics a good idea too I will mention that to hubby when he gets home!
I personally wouldn't use the play sand because it's much finer & tends to have an extremely high silica content which can cut just like glass.