Well ok, if I were building something like that, I’d start with a truncated conical frame of straights like a teepee, probably eight struts about 4”x2”, to set the basic shape. Fix it to whatever base you envisage, this thing isn’t mobile.
Then I’d get a spirit level or laser level, and mark level lines on the struts at vertical intervals.
So.. O72 circle has a circumference of 226” (72” x 3.1417: pi) at 8% grade that’s a total rise of 18”. Divide that equally between the struts, mark it relative to your starting level and that’s your first grade section. Cut a 3/4 circle of about 4mm ply to suit the track width and pin it to the rising struts (you’ll need cross members as it rises). That’s your track bed. The reason you advance in 3/4 circles is that if you don’t, the track doesn’t form a spiral and the geometry gets complicated.
Now repeat the process for the next diameter. If that’s 60”, the circumference is 60 x 3.1417 = 188”, total rise at 8% = 15”
Repeat as required. After the NEXT diameter, the track is 18”+15”+12” from the floor, 45” or 3’9” which is probably enough? The tabletop will be about 44” inside the tracks, you have room to transition to O31 or O36 and run onto the tabletop. Cut the struts to level and insert the tabletop, say 6mm ply?
Test fit the track and make sure the train doesn’t snag or stall anywhere.
As you are going for a once-only effect, drape it in green felt which you should find at any hobby shop or haberdashers. Staple the felt enough to hold it in place. Spritz it with white, glitter and generally go for a frosted appearance - you don’t need a tremendous amount.
Fix the track to the trackbed, and decorate as required.
Run train.
This is all based on a shop window display I built some years ago, for a local Scout troop. It isn’t modelling, in the strict sense, and it DOES require you to be a little bit handy with carpentry, but it’s fairly quick and cheap.
Any use?
