If you mean you want it to be 27" wide, I don't think there are any that width. You could create a shelf for a 24" one and use filler/face frame to make it look built in (someone in the kitchen forums did a velcro-attached frame that's easy to remove and looks very permanent).

Also, there are several 24" wide micros with mounting kits available, like the GE spacemaker or the Panasonic OTC micro. They are all more than 12" deep, even though designed to mount under 12" cabinets, to give the door room to open. But then you still have the problem of needing filler at the sides.

You can also get a trim kit custom made from microtrim, if you think it's worth it. (To me it's a lot of money for a not-great look, but they're the only company I can think of that does built-in type framing for 27".)

The GE Spacemaker II (model PEM31) can be hung under a cabinet or mounted in an upper cabinet with a trim kit. GE makes both the hanging kit and the trim kit which will fit in a 27" cabinet. I think the MW itself is 24" wide but will look fine hanging under a 27" cabinet if you center it. I have the previous model (JEM31) mounted in an upper cabinet with the trim and it looks great - we did have to recess it into the wall a little to make it flush with the front of the cabinets, but if you're going to hang yours, you won't need to do that.

By the way, I've been using the MW for nearly 8 years and it's still going strong.

Hey, thanks for that, weissman. I looked and looked for one with a 27" trimkit and I guess I was looking at the wrong sites, because they all said 30 for that one.

Thank you for the leads... and yes, a 24" wide one would work fine. (I just wanted to specify that I don't have a 30" upper to hang the MW under.)

I'll check out the GE and the Panasonic ones.