This took longer than I anticipated. The obvious TR shelter made from Peco 2mm walling and a couple of Wills sheets. Because it's an open box there is twice as much to do, not only the 'lining', but bringing it out to the required thickness on the two edges. Details are the usual bits of strip and 1mm micro rod for the pipes. I'm sold now on the Gordon Gravett method of gutters from sheet material with a rounded face and just a lick of black paint to suggest the groove.