The Vet cannot put the lost nail back on,..
So,..I would simply keep the wound clean (by GENTLY bathing the foot with warm salty water on a regular basis)...and please bear in mind,..it fecking hurts like hell,.so do go careful...
Your dog will hopefully grow another nail,..but,..it will take several months..
However,.you can still run the dog as soon as the wound has healed,.. and a small hard, 'start of a nail' become evident.
Personaly,..I never try to cover this type of wound...
All the best,.CHALKWARREN...