A benefit of building a large pet door is that the weight of the door is not a critical factor. A larger animal is quite capable of opening a door that a smaller animal could not manage.
Step 1 - Measure Your Pet
Before designing your door, measure your pet so that you will know your size requirements.
Step 2 - Examine the Door that the Pet Door Will be Fitted Into
Check the door into which you want to install the pet door to ensure that there is enough room in the bottom panel of the door. It is also a good idea to check how sound the door is. A large animal passing in and out is going to create extra strain on the door.
Step 3 - Mark the Outline for the Door
Mark on the bottom door panel the outline shape of the door. Cut the hole in the panel by drilling a hole in each corner and using a jigsaw.
Step 4 - Check That Your Pet Can Get Through the Hole
Before going any further make sure that your pet can get through the hole. Testing at this stage will not only confirm that the hole is big enough but also introduce your pet to the door at an early stage. This can make it much easier to train the pet to use the door when it is finished.
Step 5 - Make the Pet Door Hinge
The easiest way to make a pet door hinge with motion in both directions is to use a strip of very heavy duty polythene. This material is remarkably durable and does not tear easily if you ensure that the grain of the polythene is vertical to the door.
Step 6 - Find the Grain in the Polythene
Polythene sheeting will tear in one direction only. With a piece of polythene cut off the piece you will be using for the pet door hinge. Hold it between two pairs of pliers and try to stretch it. If the polythene does stretch you will see that it will do so from two sides of the stretch. The grain is at 90º to the direction in which the polythene stretches.
Step 7 - Frame the Hole
Using strips of wood build a neat frame around the hole you cut for the pet door. For neatness it might be best to frame the inside and outside and connect the frame to the door with adhesive and small nuts and bolts fitted with large washers. You will need to clamp the wood to the door and drill the holes for the nuts and bolts
Step 8 - The Top of The Frame
Make sure that you place your strip of polythene along the top of the frame and that it is held in place by the frame and adhesive.
Step 9 - Attach the Door Flap
The door flap will be attached to the polythene hinge with adhesive and whatever connectors are suitable. A thin plywood door, for example, could be attached with adhesive and staples from a stapling gun. Make sure that the flap is free to move backwards and forwards through the maximum arc. Note that larger animals work better with a rigid door flap because they can see outside as soon as they raise the flap. A flexible flap will cover their eyes until they are through the door.
Step 10 - Finish Off
The door can be finished by grinding down the ends of the nuts to the level of the bolts. This makes for a much neater finish. Seal and paint the woodwork to match the door.
Step 11 - Fit Brush Strips
Minimize drafts by fitting brush strips around the inside of the pet door frame.
All you need to do now is to introduce your pet to the new way in and out of the house and fit a security flap that can drop down and be locked against the door.