I got an idea to do this after sleeping in the boat with 2 kids and wife. More ventilation without rain intrusion...
All 3 portholes were sourced used off of Ebay.
The easiest install was in the head area. Some Macs don't have any windows there, some have a 4 inch square slot. In the process of replacing the tinted plexiglass strips down the sides of the cabin, I enlarged the head window area to accept the porthole and deleted the forward strip. The inside and outside walls of the hull and liner are right next to each other, so creating a new hole was easy. Also, I found this to be a good time to enlarge the two aft galley window holes, letting in a lot more light. I reset the plexiglass strips with RTV. It worked for 30+ years from the factory, and was a bear to remove.
The portholes in the footwells of the cockpit were a little harder to do. This is because the upper outside wall is spaced away from the liner. I cut the hole with a jigsaw, and the blade wanted to wander when cutting two surfaces. If I did it again, I would have made a template and routed the edge with a handheld router. When I placed the portholes, I used a caulking gun to flow substantial sealant between the two separated surfaces, to keep them supported. Initially, I assembled the portlight, then I came back and set the final torque I wanted, compressing the inner ring of sealant.
The increase in light is a wonderful thing if you camp on your boat. The ventilation makes the aft bed a very nice place to sleep. Ventilating the head keeps the entire crew happier (I also have a solar fan above the porthole to keep things moving).
The potential for water intrusion through these three holes is there, but is made acceptable by responsible use of the openings. The benefits outweigh the risks for me. Ho wever, in a complete knockdown, on a hot day when I was below, I actually did have water come in the head portlight.
I am not sure if I am right, but I think that an opening window is a porthole, a non-opening one is a portlight. I saw a set of portlights mounted in the vee berth area that really made the area more pleasant even if no ventilation. The portlights were mounted below the rubrail and in the area of the bow that leans in, so not apt to be contacted by docks etc. I may do that too.