It sounds like you've done what I did when I built my present coop - finished all walls without an opening for a chicken door.
I determined where my studs were and plunged the skill saw into both the exterior and interior of the wall. Then I finished the corners with a jig saw. It was a little more difficult since the wall was insulated with fiberglass - so I had to make the skill saw cut, very shallow. I probably used a metal cutting blade on the jig saw and just "plowed" on thru the fiberglass.
I could then cut & pull out a chunk of fiberglass (where oh where was my planning?) and force in a short piece of board, drive in some screws to hold it, and finish the frame.
The door is really very small but I justify this by thinking that it helps keep the coop a little warmer in the Winter. A hen needs to lower her head and her wings touch the sides as she passes thru the door. The current residents don't seem to appreciate it as much as the other chickens I've had in the coop. They grew up in the outdoor side of the coop - what I see labeled as a "sunporch" by some coop builders. It has a board floor and a roof but 2 sides are wire.
With the cold weather, I've opened the chicken door and am teaching these pullets to make use of the other half of the coop. They've been very suspicious of this opening that suddenly appeared in their home after months of comfortable living and sleeping on the "porch."
I moved their feed farther and farther into the insulated interior. Then the waterer went in there. Right now, I'm putting a treat inside about sundown and when they go in, I close their chicken door behind them. I've been a little late a couple of times and have had to pick them up off the outside roost and push them thru the door into the lighted interior - one at a time. They're sleepy, so it's an easy process.
Soon, the birds will forget they ever roosted on the porch but I'll need to close them in every night as protection against the cold.
Steve
