Cutting curves in plastic

I'm about to have to fit a sand dome on a new boiler, and to cut down the smokebox saddle on my new project loco. Both the dome and the saddle have to fit a curved surface about 1+" radius, (2.375" diameter actually. It's a ten-wheeler conversion, so the dome is moulded in to the top of the old boiler.)

I can think of a few ways of doing this, but I'm wondering if you clever guys know any other way besdies the following:

- using a coping saw (hand version of a band saw,) carefully cut to approx the right diameter and then file or sand smooth,

- using a large curved file, just file away until it fits,

- buy a grinding bit for my drill and grind the plastic away.

How would you make a sand dome fit a new boiler?