It was becoming clear that the only answer here was to produce my own. I had a very nice dome casting from Laurie Griffin which was a perfect fit on the cylinder but was much too tall. I considered cutting a piece out of this (complicated by the fact that the dome was hollow) but it really required a lathe, which I didn't have. However, it occurred to me that I could use this as a pattern to produce something which could be more easily modified.
The idea was to temporarily attach the dome to a piece of plastic pipe, the same diameter as the cylinder, to make a pattern and then use this to make a copy of the dome, all in Milliput resin.
The problem was that not only did the resin never fully set but didn't want to be parted from the pattern. This was the first time that I had used this material and it may well be the last. Maybe I didn't mix it enough, but it was firmly stuck to the pattern. I was beginning to think that the old aeromodelling standby of epoxy and microballoons might have been a better idea.
I looked around for some suitable castings but they were only available as a part of a complete motion set and cost far more than I could afford, even if the were the right type, which I couldn't establish. The only thing for it was to attempt to use what was supplied plus a bit of scratchbuilding.