I’ve tried with the béchamel sauce and adding cheese but I just cannot get that creaminess and cheddar flavor like Stouffers has. Any ideas?
Don’t use real cheese. 
Seriously, I would imagine they use some generic version of Velveeta or ,if they really want to spend, American Cheese.
I just made “real” mac&cheese for the first time and it turned out awesome. I went ahead with the bechamel and used 1/2 velveeta and 1/2 extra sharp cheddar. Don’t forget to add some dry mustard and perhaps some cayenne for depth of flavor.
Also note I added crumbled bacon to half of the recipe and it was to die for-- and I normally don’t even care that much for bacon (yeah, yeah, sacrilege, etc)
This is the best mac 'n cheese I’ve ever made. Don’t skimp on the cayenne.
Mustard is a key ingredient. If you don’t like the kick of cayenne, garlic powder makes a good substitute.
I like a squirch of Worcestershire sauce and a dash of dry mustard in mine. Brings out something that people seem to love.
Warning: I have not tried this, but the source is the guy who did all those Copcyat recipe books, so hopefully it’s pretty faithful to the original.
John Thorne has some pretty good tips collected in an essay printed in one of his books – IIRC he used far more cheese than most recipes call for.
I’ve used cheese powder bought in bulk in the past for the Kraft-style stuff, but I don’t think that’s the Stouffer’s style – with a good quality dried pasta, it’s edible when drunk and it’s late.
Dried mustard is essential. One time, though, in a pinch, I added a little spicy brown mustard (from the bottle) and it was pretty tasty. Another option, if you don’t like cayenne, just a tiny smidge of horseradish or wasabi. A totally different kind of depth and heat.
What is the ingredient in the best M&C’s (such as Stouffer’s)that imparts the slight hint of stomach bile? I read about that in a serious article on the subject severalyears ago but did not retain it.