I can't comment on the WC-122, but from the 2250 user's manual on planning a custom installation comparable to the WC-22:
"When planning a custom installation, allow adequate spacing between the model 2250, cabinet surfaces, and other components for adequate ventilation. To install the Model 2250 in a custom cabinet, cut an opening 16-7/8 inches wide by 5-1/8 inches high. Since the front panel of the Model 2250 is larger than the cutout, it will neatly hide the edges of the cut. Remove the plastic feet from the bottom of the unit and slide it through the opening. To support the weight of the Model 2250, adequate bracing across the rear of the cabinet must be located to provide contact with the rear of the unit."
The lip of the brushed aluminum is 1/4 inch, so you would need 1/2 inch thick wood to cover up the lip and to have the fancy angle molding thing like on the WC-22. That means that the total dimensions would be:
Inside:
W: 16-7/8
H: 5-1/8
D: Over 15 inches, but measurements may vary.
Outside:
W: 17-7/8
H: 6-1/8
D: Varies. Measure your unit.
To get a general idea of the fancy stuff on the WC-22, look at eBay listings with the top and bottom showing. Ventilation is necessary!
Disclaimer: This may not be 100% correct, especially the outside and depth dimensions. However, if I were to make a case, that is what I would use.