Three tedious days of trimming, which I bet is precisely three times the amount of time it would take a non-perfectionist to achieve more than satisfactory results, but nonetheless, the top now sits just right.
I'm told that this is by far the hardest part of the installation, but we'll just see about that. Plenty of steps remain that hold potential for me to drag out interminably.
Work has intruded this week, so about the only other thing I've managed to accomplish is flipping the top over and giving the interior a good bath. See, there's a wax residue from the molding process that apparently conflicts with the water-based contact cement I'll be using to install the insulation. Solvent-based contact cement would avoid this step entirely, but alas, such luxuries are banned here in California.