If i look at this photo, the opening of the roof should be relatively flat, and just about as wide as the hump on the middle of the boiler, maybe a little less.  I'd say it needs to be roughly 1.3 times as wide and 0.8 times as deep?

There also seem to be either openings or ridges behind the sliding roof on either side at the back of the roof as can be seen in the pic attached

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