They're really 6" deck plates that are used in all kinds of boating applications. They're sold by most well equipped marine supply houses.If you can call up the on-line catalog for an outfit like Boaters World or West Marine I would think that the hole requirement would be stated for each different size deck plate they sell. Why CLC doesn't do it beats me. Be aware that there are two different kinds. A snap-in model and a screw-in.
I considered using one some time ago and took a look at one. It can be installed from one side but be aware that your bulkhead has to be thick enough so the screws have something to grab on to.
Charlie

: Has anyone installed a 6" access port like those sold by CLC? I'm
: nearing completion of my Night Heron and want to put one in the rear
: bulkhead so that I don't have to cut into the deck. I am about to install
: the bulkead and want to cut out for the access port although I don't have
: it yet. I need to know the rough opening diameter (6"?) and be
: reassured that I can do this all from the cockpit (don't need access to
: the back side of the bulkhead) so that I can cut for the port and install
: bulkhead now, then install port when it arrives? Any thougts would be
: appreciated.