Like many horse owners I have struggled with finding and keeping enough rear leg straps around for my horse blankets.
These are made from the same materials that your originals were. Soft but sturdy elastic to provide give and not rip out the Ds on your blanket.
With a loop on one end to attach to one of the Ds and a clip on the other end with a slide in the middle to make them as adjustable as they need to be. Which makes these easy to install and adjust for your blanket on your horse.