Hatch removed. (I’ll put it up for sale on craigslist. Those things aren’t cheap — maybe someone needs one.)
And I just wanted to thank whoever installed this metal hatch way back when for not using 3M 5200. I think I would have turned to strong drink had I found it so. Dolfinite was used instead, which made removing the flange easy with just a tap or two from a mallet.
Now the fun part… returning the opening to it’s original rectangular size and shape, building the hatch and coamings, installation and finishing.