which non dairy substitute would work best for crustless quiche? In particular, would like to slice and have slices hold their shape.

Nut milk, rice milk or, for an Asian fusion quiche, coconut milk.

How about making Southwest Egg Muffins instead? They're portable, can be made ahead of time, and contain no dairy: http://www.healthline.com/health-recipes/southwest-paleo-egg-muffins

Another option is a Baby Spinach and Mushroom Frittata, which also contains no dairy: http://www.healthline.com/health-recipes/baby-spinach-and-mushroom-frittata

I find soy milk is a good non-dairy substitute