It sounds with that cabinet size you don't have a lot of choice. The link you provide appears to be a great price of $250 (from $400). That sounds like it's worth a shot for you and seems to have the dimensions you need. A reviewer mentioned a side plug you might have to worry about, but if the opening is 20 3/4" then the inside should be much larger and not a concern.
I like the remote and the onscreen display. My Dimplex does not have either and means we guess a lot and get up to adjust the heat a lot, until we get it right. I hope the display goes away or else don't let it bug you that it takes away from the ambiance.
You'll probably want to dedicate a 15 amp circuit to it (you might need to run power and install a receptacle). The specs show it has comparable BTUs to the Dimplex, and we are happy with our Dimplex in our Rear Living Room and is a good supplement to our propane central heating.
If I really needed a fireplace and had the size limitations you had, for that price I'd give it a shot. Check their warranty and go for it (I hope the shipping is not so bad). Dimplex can't be the only game in town.
btw - they do have a website
www.classicflame.com you might visit for more information.