I have a Mk III rather than a Mk V, and I installed an opening port some years ago.The installation should be similar. I found a port with the same height as the opening but it was just slightly longer than the opening (which had oval shaped ends)- so a bit of cutting was needed thru the deck and the liner. I remember making some "filler" pieces out of marine plywood to fill in the space between deck and liner, putting them around the perimeter of the opening so that the inner and outer flanges of the new port had something solid to butt against. Use a good sealant and " Bob's your Uncle" .
Clare Jordan Aragorn