If the work piec is large enough, and not very intricate, I would use a jig saw and a fine tooth blade such as a metal cutting blade. That would yield a smooth cut.
Are you talking about things like yard art that we see in the yards at Christmas time? Jig saw for sure. The way I see it, cut it to the approximate size and shape as needed, sand a little here and there. You are the artist. Make it look good and no one will every know you were off by 1/4 inch somewhere. :-)
That is similar to installing cabinets in a house with crooked or uneven walls and unlevel floors. It is the installers job to make everything look good.
Good luck. Post some pics of your stuff when you can.