This photo shows the floor screwed to its MDF support base and clamped in the mill. The tool I’m using here is a 6mm end mill which came in a set. As these tools are designed to mill steel, working on a polyurethan casting is hardly going to cause it much trouble. The mill’s table is shown at just about the end of its travel, this is why I had to stop half way through the job and shift the casting down so that I could mill the other end. Once this side was done, I unscrewed the casting and simply turned it around and milled along the other side.
Morpeth In O-Scale 
Not to detract from your solution but having to go to this length to achieve a successful kit result makes me wonder if building from plans using traditional materials like balsa, ply or brass would be more straightforward. Maybe even MDF?
You may well be right Stephen but then I know that without the kit as a starting point I wouldn’t build the thing. It’s not a matter of going to these lengths, it’s more a matter of using the tools at hand. I worked for 20 years with files and emery paper, I now have a mill: it would be a shame not to use it because other people might feel I go a little far in getting the results i want.