I did this on the roof of my 2 story Mt. cabin. I have a deck on top and wanted inside access only. Before I built the deck (sitting on sleepers) I cut the hole in the roof and built a curb. I took careful measurements and decked over the hole. I then cut my access hole and built the hatch. This way the deck boards line up when the hatch is closed (you can use hidden hinges, I didn't as the hinges are next to the deck rail). You can see the finished hatch over hangs the curb allowing no water to get in. My water tight hatch weighs 70 pounds (pretty heavy to open) so I went to the auto parts store and picked a car hatch back strut that would assist in lifting. The in store catalog listed them by weight lifting capability.
