For one porthole you need two Groggy trays and one circular piece of glass or acrylic 345mm (13 ½”) diameter and 3mm thick. A glazier can cut the glass for you and he/she can also heat treat it so that it’s toughened and much safer to use. Toughened glass is much stronger and you won’t cut yourself on it if it breaks. I also used some foam weather strip.
1. Cut the bottom off the trays so that the thickness of what’s left of the two trays plus the glass equals the thickness of your door. For example if the glass is 3mm and the door 44mm you should leave 20mm of the tray (strictly speaking it should be 20.5mm but a 0.5mm gap is OK). I used an angle grinder with a thin blade to cut the trays but you might be able to use a hacksaw or jigsaw with a fine metal blade. Note that the tray is stainless steel so quite tough. File down the sharp edges.
I fixed some self-adhesive foam weather strip along the outside/backside of the cut edge of the trays. I let it stick out a little bit so that when the two trays come together the strips get squeezed against the glass and hold the glass in place. This is especially important to prevent the glass from moving if you’ve got a hollow core door. If you’ve got a solid door you can just put a bead of silicone around the glass/steel corner afterwards to stop the glass from rattling around.
2. Cut a circular hole in the door the same size as the glass. You can use the glass as a template and cut it with a jigsaw. I used a router and a pivot hole in the centre.
3. Fix one tray from one side of the door and then fit the glass and the other tray from the other site. I just glued the trays to the door with silicone but you can also use screws or even straight through bolts for a more nautical look.
We’ve had 9 of these portholes in our home for over 10 years now and they still look great. They let extra light in and let you glimpse what’s in the other room. I don’t use them on bedroom or bathroom doors but you could use frosted glass or mirror glass instead. Please note that the portholes shouldn’t be used in fire doors for safety reasons. I also wouldn’t recommend using them on an outside door unless you fix them very securely and make sure they don’t let rain water into the door.