Go with something biodegradable as we have enough damn disposable diapers in the ground at this time !
This is an old trick and it works too , something easier and more economical all the way around - coco or peat moss
you could also use wooden boards on the bottom of your holes (non treated wood!) to help retain moisture, compost & of course the water crystals are bomb-diggity.
May be some people use diapers for this purpose. But there are other many ways also You will absolutely love everything The Honest Company has delivered; I will highly suggest this company!
why not just use rockwool in the base of your hole and through out the mix. holds water very well this we all know ., seriously there are enough crap not bio-degradable already in the ground. so many ways to keep moisture in a hole it's not even funny.