i am building 3 madisound Odin MK3 speakers (see link) as my front 3 speakers.

http://www.madisound.com/odinmk3.html

however, the media center i have chosen for the current room (will change later in another house) is the Hooker Esplanad. see link below.

http://www.hookerfurniture.com/index.cfm/go/catalog.show_product/product/379-70-565/console.cfm

The location for the center channel is either perched up top (looks not so much), or hidden in the center panel below the TV (unit comes with fabric instead of wood attachment so speaker can sit behind it). The dimensions for this area are 12" (w) x 16" (h) x 20" (d).

I will have the right and left speakers (the MK3's) sit outside the media unit (to the right and left). The question that remains is what to do about the center channel? Is there something that I can either purchase or build that will fit in the 12x16 area and integrate well with the Odin MK3's?

I have two NHT IC3's installed as my surround speakers, and all 5 are powered by an ADA MPA-501 amp (www.ada-usa.com).

Any help from you guys would be great. I was thinking of the NHT Classic 3 since it fits, but I am worried the difference in quality will be quite noticeable. Also I believe the speaker should be horizontal (width > height) (true center) for the best sound. But my space constraints are killing me....

I vote:

Another Odin for center channel -> perched up top, then slighty angled down
at the listener using a custom made bracket.

agreed, this is the ideal solution for a match, just the aesthetics are not so much. i'm making 3 of the speakers so the extra speaker is not the issue.

i'm just trying to see if there is a way to get the sound quality i know and love, and also achieve some sort of aesthetic middle ground so it doesn't look like an ugly solution.

I vote to make the center a sealed speaker. You can get a nice 0.707 w/ 11L per driver. This will shrink the cab by ~35%. You can cut the back off the box and make it say 9.5" deep vs. 14" before.

Also I believe the speaker should be horizontal (width > height) (true center) for the best sound. But my space constraints are killing me....

The advantage of the MTM configuration is that it exhibits superior horizontal dispursion. Meaning the frequency response off axis is more like the on axis frequency response.

Turning it on it's side defeats the purpose of the MTM. If that's the case you might as well turn the center into 2 drivers, cut the internal volume in half and muck with the xover to increase the gain to the mid/bass.

Personally I would mount them all above the entertainment unit all in vertical position all at the same height. This will make the wall look unbalanced (ie top heavy) Maybe put some plants underneath on either side, or a small piece of furniture to balance the wall out. (yeah yeah my gf makes me watch home&garden)

Have you thought of doing your own design? You could build a small bookshelf speaker to fit in the shelf under the tv. Not optimum but at least you won't see it.

If you must use a known design this is the closest I found:
http://www.troelsgravesen.dk/W15.htm There is also a ribbon version, though that might be more picky with aiming at the listening area. Ribbons tend to have less vertical dispersion.