For existing construction - to be retro fit into an existing drywalled 2x6 wall. It is white powder coated and does not need to be painted. Please review the instructions at this link before purchasing as there may be obstacles and special tools needed. Since the majority of "mechanical walls" or walls behind the washer and dryer are deeper than 3.5 inches to accommodate the 4 inch dryer conduit and other mechanical fittings it is likely the Model 500 Retro Fit will be the most appropriate and efficient option. The Model 500 exhibits a round hole or port.
 

• 22 gauge aluminized steel, click here for enlarged image
• Powder coated (similar to finish on dryer appliance)
• Top port measures 4 1/8" with very slight oval shape
• Distance from mounting flange to rear of box is 5 1/8"
• Overall measurements: 21" h x 11 1/2" w x 5.12" deep
• Inside measures 9 1/2" wide x 18 1/4" tall
• Two mounting 3/16" holes on bottom flange
• Curved shoehorn like assistance tool included (see video)

Links:
Pictures of other installs  
Dimensions  
Instructions
Alternate Installation Method Video