My house has a retrofitted upstairs with hatches throughout the rooms leading to a crawlspace AROUND the rooms. They were fitted with these plywood drywall contraptions when I bought the house. I purchased these doors to replace those hatches. They screw in easily, two screws on each side. The screws on the hinge side go in easiest if you are on the inside of it and close the door. That will not always be feasible so you may just have to drive them in and force the door through. A small cutout in the hinge side where the screw holes go may be useful. In my case I just opened the door and forced the metal to skip over the screw heads and now it's fine. This is just metal. In my case with the attic I have to add insulation to keep the attic air from seeping through the cracks and crushing my HVAC. This is simple for me. I added 2 inches of foam board insulation to the back of the door using construction adhesive and I added simple adhesive foam weatherstripping around the inside edge of the frame to cut off air flow. Problem solved for maybe $30. All-in-all, installation was straightforward and it did exactly what I wanted it to do. I knew I'd need to insulate going into the project so that wasn't a big deal for me. One thing though, the locking mechanism is non-keyed and is welded in. If you hope to replace the mechanism with a key lock, you will not be able to with this product.
James 
