Reply by renners

I recommend using winding sticks to check there’s no twist in the stiles, also use them when doing a dry fit and gluing up to make sure there’s no racking.
Check the shoulders on the rails – if they are not quite level you can glue in a warp when clamping. Make sure your assembly area is perfectly flat when it comes to final assembly.
Once you’ve got your components to the right dimensions, check them all again for being straight and square.
If you find the stiles develop a twist or bow, you might consider laminating the stiles. This is something I have had to do previously on walnut cabinet doors that came off the planer perfectly, but went totally out of shape within a couple of days. Hopefully you won’t have this problem seeing as the door is so sturdy. I think you’re right to put in a veneered plywood panel.
The only thing to look out for on the veneer is the way its knifed and put together. If there’s any sapwood in the veneer and the pattern repeats it can look like the front of an Edsel.

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