I am istalling a shower in my basement, I have a shower drain opening, a 12 x 14 opening in the slab, a black plastic box was fitted into this opening before the pour, the box is full of gravel. My question is, Do I remove the box and pour in concrete to fill in the gap or cover it with a piece of concrete board and pour my shower pan on top of it?
I'd recommend only capping the trap hole with a fairly thin layer of concrete...Only an inch or two. Fill the hole with gravel around the trap. That way you're not encapsulating the trap in concrete. If there's ever a reason for you or a future owner to need to get in there to service or replace the trap, you don't have to destroy it to get it out.
(you still might have to destroy the shower though )