I am finally replacing a temporary on-slab shower base in my lake garage project. I have the trap installed in the large block out area which is surrounded by 2x6's, sand and slab insulation. I have a Jacuzzi brand plastic shower base and want to set it in mortar to make it level, firm and quiet. My question is do I leave this block-out in the slab below generally open so the trap could be more accessible (dread that thought) or is it common practice to fill that area in with concrete patch or mortar too? Should I pry out the 2 x 6's too?