I have an issue which I need to fix. 

I created a recess for a sillpan of a large sliding door, which connects with what was to be a brick-ledge (we switched to metal siding installed over cascadia clip system with rock wool external insulation). As the sillpan is 10″ wide, I added a 6″ angle iron welded to a 4″ square post to allow for a secure fixation of the door frame. I suspended the angle 1.5″ above the bottom of the sillpan recess as it was uneven and I thought I would have trouble sealing it. I then added sheathing. The space between angle and sheathing is filled with rock wool insulation. 

So, now I have a 1.5″ deep 6″ square cavity, between the brick ledge and sillpan/door frame which I need to fill and seal. What is best way of doing this? Spray foam insulation? Concrete through a hole in the sheathing? I know I should have done this earlier because now my access is very poor – but I procrastinated and now have a more difficult situation… Please help, thank you.