For these kind of repairs, acrylic or other plastic glue that uses ultraviolet light as a hardening accelerator is ideal: you can shape before applying the UV light, the material hardens instantly with the light, then you can build up some more, file, shape some more, until the shape you created is ideal.
The new material will be as hard as the original plastic or acrylic: There is no need to wait endlessly for the material to dry. It's the same bonding process we are familiar with from dentistry: the ultraviolet light hardens the bonding material exposed to the light with in seconds.
