I have not had the need to make one from scratch - but when I directed Man of La Mancha. (Quixote's Rossanti-horse & Sancho's Ass/donkey) I obtained 2 latex full horse head mask - adult size - from a joke/costume supplier. They were realistically moulded & worked well. I think they were originally mass produced & inexpensivly turned out like biscuits,  from your part of the world Eagle.
I modified them by cutting out most of the eyes & inserting a gauze/net (black tights/pantry hose) under the chin/cheek, to allow for ventilation for the performers, as they were tap dance/equine style. Because the masks cover the head fully, they do tend to get very hot without any vents fitted. Also as it was supposed to be improv, done by 2 of the prisoners, I sewed hessian (burlap) around the masks neck skirt, so it draped over their shoulders to the their knees.  I hope this helps?