Hi spindoctor Welcome to the forum. making a planter mold sounds interesting. You will have to show us your planter when you finish it.
I havn't made anything like that, but I did make a mail box out of cedar that looks like an old steam engine. It is all wood except for the wheel rods which are made out of brass. My wife would not let me put it in front of our house, was to worried about it. I designed it myself, rough sketch and then put on autocad, which I am afraid to tell how long I spent just drawing it up. You gave me an idea. I could increase the scale of the train to the size of a planter for flowers or . . . I would have to try to make it simpler than the mail box. The boiler section was made of 10 sides, like and octogon type thing. It only needed to be 1/2 barrel, but you have to make up the whole tube so you can glue it up. I used a technique to make the 18 deg. sides meet flush. I needed this since I wasn't doing any joining and because I wanted to turn the outer diameter on a lathe, to make it look better. I guess I shouldnt not have done it on the lathe, since I must have the smallest lathe in the world and I never used a lathe.
One other thing I would like to say is that I live in Milwaukee, Wis. but my daughter lived in S.D. for 8 years and one year in Santa Monica. I visited her last year for a month and really loved walking everyware. I walked all the way from her cottage once to Venice beach once and a bit on the sand-very tired, so I decited to take your talking Blue Buss back. Was quite a day