When you make a crop bra, the square of cloth supporting the crop should be of a sturdy, non-stretch fabric. I like canvas or cotton duck. The straps can be stretchy, and you will be adjusting the crop several times until you get it just right.
The way you do that is to check the crop in the morning after the hen has rested all night and not eaten or drunk. If the crop still has material in it in the morning, you need to adjust the bra so it holds the crop a bit higher.
It could take several days to get it just right. Don't be in a rush. The hen won't die during this adjustment period as long as she's still eating and drinking and pooping.
The way you do that is to check the crop in the morning after the hen has rested all night and not eaten or drunk. If the crop still has material in it in the morning, you need to adjust the bra so it holds the crop a bit higher.
It could take several days to get it just right. Don't be in a rush. The hen won't die during this adjustment period as long as she's still eating and drinking and pooping.