Kerstin, it is amazing how quickly sand dries out. I only dried it on a fire because I was being being a bit ott and wanted ny chooks to be happy while I went away for a couple of days. You can always try a kiddies sandpit?paddling pool type thing. the plastic type with a lid, then you can put the lid on when it is very rainy( most of the time at the moment) you often see these on skips and at carboots.
Chickens are very addictive, so watch out, you'll end up with more than you first thought you needed.