How to Tell If a Replacement Door Will "Work"

I want to replace an odd-sized lauan door leading to our cellar with a used but solid wood door I got at a Habitat for Humanity store. I have a cat who likes to go outside in warm weather, and I'm getting too old to stay up all night with her :), so I thought I'd build a "cat door."
The odd-sized original door is 1 1/4" thick, 29 3/4" wide, and approximately 80" tall. I chose the $5.00 wooden door because its width matches the lauan door exactly. However, it's only 1 1/8" thick, 74" tall, and has hinge placements and sizes in different areas than the lauan.
I filled in the hinge areas with very thin panel board and can chisel out new areas. What I'm concerned about, however, is possibly doing all this work only to find the door out-of-level or (since it's panelled) unable to be hung with the mullion and stiles at 45 degree angles to the floor. The door has ancient skeleton key holes and needs to be puttied and painted, and I don't want to waste putty or paint, or TIME! on something that won't work.
So I just thought I'd ask if there are any tips for knowing in advance if a particular replacement door will work in the area you need it. Thank you.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Method 1: Hold it in the opening with shims underneith and verify directly. There must be a reason that is difficult or I think you would have done that already.
Method 2: Careful measureing. Check the diagonals to verify it is square
Method 3: Make a template out of cardboard. This seems like overkill IMO and if you don't have a big box to get rid of, might even cost more than the door did.
Method 4: Lay the old door on top of the new one and match it up. Seems easy, cheap and reliable.
You can probably plane some off if it is a bit too big or if the frame is funny but if it is too small you are out of luck. Hopefully this wasn't already done to extremes.
Even a new replacement door would require almost as much work as your putting into this one. (the handle would be easier)
You lost me with the 45 deg angle to the floor reference.
You can still use the hollow door, you just need to glue in some blocking around the cutout, the trim on the cat door will cover the rest.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks, Pipe Down. The reason I didn't just compare (shim) it to the frame is because the lauan is at the top of a steep flight of cellar stairs. While I can manage (and have repainted and rehung) the lauan by myself, the solid wooden door is too heavy for me to handle alone.
Method 4 is the most logical. But what did you mean by measuring the diagonals? Particularly because the cat door is paneled (that's what I meant about mullions and stiles perfectly perpendicular to the floor), I think this "issue" may be pertinent.
I just thought buying a new door would reduce stress in case something in the original door got destroyed! I tried to come up with a professional method of cutting an upside-down "U" shape with my circular saw and jigsaw, but it got pretty messed up, and I'm going to make liberal use of duct tape and padding so the cat won't get hurt.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

If the old door had been planed a lot to make up for a crooked frame it will not be square. The diagonals I refer to were the corners of the door itself. If the top-R to bottom L measurement is not the same as the top-L to bottom R then it is not square, you may need to plane it more to make it fit.
You lost me on the paneled cat door thing

A sharp razor in a box knife with multiple passes will cut Luan cleanly.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
snipped-for-privacy@yahoo.com wrote:

Do your cat, and the neighbors, a favor and keep it indoors. Simplest solution :o) That way, the neighbors won't have your cat's poop to contend with nor the other strays that are attracted by it. Your cat will be safer and possibly healthier, too.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Norminn wrote:

Thanks for the advice. I live in the mountains; the cat is very old, very spayed, and very innoculated. I'm sad as all get out, in fact, that she's not as interested in her old haunting grounds as she used to be.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.