I just found a book on Amazon.com called "Quick and Easy Topiary and Green Sculpture". They show some sample pages from the book and one of them includes how to make a form for a Chrtistmas Tree topiary. I haven't read the book, but you can at least check out the cool photograph and explanation on the sample page and decide if it is worth doing or if it answers your question.

I have seen a rosemary topiary on Martha Stewart's homepage. She was showing how to make one around winter time, and it was one of those topiaries to put on the table, something about 12-16 inches high, or maybe a little higher. You might want to take a look at it. I think it's MarthaStewart.com, then choose the gardening or home decor tab. Good luck! I know it will smell wonderful.