Two of Thor's favorite girls are now needing saddles. I made one saddle so far, to check it before I make any more. (see below) I sewed them 7 1/2" long, 4" wide @ the top, and 8" wide @ the bottom. To know how long to cut the elastic I measured one of my girls from the base of her neck centered then under the wing to where the side of the saddle would be on her back. I came up with 9", so I doubled that and then deducted 2" (allowing for the give in the elastic). The saddle slides all over her back for a minute and then she shrugs it off. What am I doing wrong? Do I need to cut the elastic smaller? Also, the elastic is free flowing thru the hem at the top. Do I need to anchor it so it doesn't slide back and forth. TIA.