Depending on design specifics of your individual building, you can find manufacturers online who ship ready-made, ready-to-install steel or plexi-bubble hatches. They are typically designed for big buildings and warehouses, etc. They are not cheap to acquire, of course, and you’d likely have to modify your structure to accommodate sizing specifications. Or, if you have a basic rowhouse-style hatch like me, it’s pretty easy to do it yourself. I built a wooden box of 2×4’s and 1.5-inch thick plywood, added hinges and sealed it along the seams — both inside and out — with all-weather silicone. Then I screwed a sheet of stainless steel (to bend or fold the steel to fit the dimensions of the box, just score some lines where needed and bend — pliers can helpful here). After stainless-steel sheet was screwed on, I got a silicone whose chemical composition was compatible with roofing tar. I dabbed that silicone on every screwhead and folded seam. Allowed to dry for about 48 summer-time hours (obviously kept an eye on weather/rain, etc…). Then I slathered with roofing tar. The finished hatch is lockable with basic eye-hooks on the inside and it was easy to install an alarm sensor. It was pretty cheap and relatively easy to do. Two and half years later: Still fully functional, sturdy and absolutely no leaks.