We used something like this that had a hard plastic loop thingy behind the heel so the heel was suspended with no pressure. The name escapes me-I think it starts with an L
We also used a foam boxy thingy that had circles cut out so most pressure points were suspended.
We also used a foam circle that was wrapped about the lower tibia so the heel was suspended.
I haven't seen sheepskin used except on Thomas traction splints in a long, long time.