Go to gordoncelladoor dot com.

They have both types of doors that you're inquiring about. (Sloped and flat surface installation.)

Once you've identified the components you need, go to HD and order them. They show which measurements to make. As in, if your opening itself is 48' but the outside dimension including the walls is 56', then the RD-3 would fit @ 52' wide. Then, determine whether you need the RD-3 with a 12' extension or an 18' extension to match the necessary length. Now that you know the the components, (RD-3 and whichever extension, your trip to HD will be much more productive.)

You could also try www.bilco.com
They are another brand that you can get either through HD or Lowes or another local retailer. They're a bit more expensive.

Here is a link that might be useful: Gordon Cellar Doors

any ideas on how i can build some doors out of PT wood? the prefab units are a little more than expected.

Did you price out Gordon? They were about half of what Bilco was for my door.

The problem with wood is that it will sag and you'll have to seal it. To prevent it from sagging, you'd need framing. If you make doors with frames, they're going to be too heavy to lift. It's hard to beat fiberglass or metal for the application.

Good luck.