I made a trip to "Homebase" the poor British relative of the USA's "Home Depot" and (surprisingly) found what I was looking for - namely polystyrene ceiling coving. Very 1970s but just the job for the squat octagonal tower comprising the North-Western corner of the complex. At £1.79 for about 4' it is a steal. I simply hot-glued four sections together to form the octagon. All other parts were lying around the garage (much to the disgust of the lady wife) and thus cost me nothing!
|Crusader Austrian for scale reference.|
|Bird's eye view.|