Chicago tools 4 amp reciprocating saw.
sawzall, sawsall, saws-all, sawz-all, saws all...
I've owned several reciprocating saws throughout my life. It's a versatile tool. It's a dangerous tool in the wrong hands. Binding the blade up and having it kick back can shatter it in several pieces, all of which get sent flying through the air like hot shrapnel. I know. I've done it before. Used carefully, it's one of the handiest power saws ever invented.
I installed a "doggy door" in the back door for Buddy and Dixie last year. It completely revolutionized "potty time." The pet door was made by Petsafe and was the largest size they made, whatever that is (read: most expensive). My back door is solid insulated foam sheathed in an eighth inch of steel on each side. It's a good, solid, heavy door, or was until I cut a big hole in it. The doggy door was a snap to install. I used a one inch drill bit to drill holes in all four corners and then used the reciprocating saw to make four straight cuts. A circular saw with a metal blade would actually have been the best way to do this, but I didn't have a metal blade. I did have a metal blade for the reciprocating saw and it did the job rather nicely.