External wall not wide enough for heat exchanger, extend out?

[PlumbBob]

Hi, I’m looking to install a through-wall heat exchanger. Walls are 300mm thick, what is neatest way to create a solid frame to extend the external wall out to 450mm? (Inside will be flush)
I’m thinking of a timber frame, but a masonry one is preferred but I think that would be too much beyond my skills! Hole needed is 600mm wide by 300mm.

 

[^woody^]

Are you sure you're reading the instructions right?

 

[PlumbBob]

Yeah, I think it’s designed to ‘hang out’ so to speak, instructions aren’t the best. I’ll try and install in a discrete location because it won’t look pretty externally.