UPDATE: well, this caused a lot of stressful thinking.  we just weren’t sure we could get it in without causing alignment issues down the road.  Even using a LFH was daunting (little frikin’ hammer).  It wasn’t the corners that were the issue, it was the sides where the side members are- along the copper staples.  I sharpened up my box plane and set it as low is I could.  We eased just a bit off of each side.  I had drilled several holes in the stiffeners to hang a net or such in the foot compartment at a later date.  We attached some ropes to the stiffeners through those holes and then leveraged the flat into place and BANG! it dropped right in to place.  🙂