I don't know if this will help you - but there is an optional drawer accessory available with the advantium. The dimensions of it are 24x3x18 and the trays fit inside. If you build a space to the same dimensions it should fit? I am ordering a Monogram Advantium also. Can't wait to use it!
Good luck.

Here is a link that might be useful: GE Monogram accessory specs

I have an Avantium 120 in a cabinet above my oven. I had the cabinet maker put a drawer below the oven for storing the tray and other items. The oven cabinet is just a shade over 30 inches wide from one edge to the other.
The metal tray is 16 inches across fits easily in the drawer below with room to spare.
This is how my set up looks:

Thanks so much for your responses! Are both trays 16" in diameter then?
I do have a drawer below my ovens but it's pretty deep. I'm trying to figure out if I can put my trays on there sides in this deep drawer or if I should configure the upper cabinet to look like this - the trays could go below the other pans.... http://pinterest.com/pin/253046072785102717/

I'm excited about mine as well! Thanks again!

Is yours a wall oven that fits between cabinets or an over-the-cooitop oven that's hung from the 12"deep wall cabinet above? The first kind can take 16"+ round trays (there's a full-width second shelf on the 240v models), but the second kind is limited to 13" round trays, since the ovens sit between 12"d wall cabinets rather than fit into a 24"d cabinet.

Our 240V Advantium with a double shelf space we built below. All 3 trays are 16", but the rack is almost 20". We also store some pot holders there.