If you want to include topiary shapes in your windowbox, but don't want to wait, try using a wire topiary frame in your windowbox and grow ivy through it instead...
Traditional plants such as box and yew can take several years to give the desired effect with a topiary frame, but ivy will produce a convincing shape far more quickly.
Alternatively, choose a topiary frame that will clip around an existing plant - so that you can trim it into shape...
New Topiary is this new technique for creating topiary forms using ivy and topiary frames. It has been developed over the last two decades in the USA, in particular at Longwood Gardens near Philadelphia, by topiary expert Patricia Riley Hammer.
Instead of training ivy over a bare frame, the topiary frame is stuffed with either sphagnum moss, or with a surface lining of sphagnum holding in a central core of compost. Rooted ivy cuttings are then planted into the frame and the shoots are pinned to the surface, into which they will root.
Hedera helix cultivars such as ‘Eva’, ‘Goldchild’ or ‘Shamrock’ are small leaved ivy's that are well suited to this method. Patricia advises that they should be spaced about 10-15cm (4-6in) apart. With careful irrigation, this method should give complete surface coverage in around three months.
For more information, you can read her book The New Topiary: Imaginative Techniques from Longwood Gardens
Topiary frames are sold from specialist shops - or you can make them yourself from chicken wire.
so you can simply clip it around your plant and trim to shape
Small topiary frames can be found at Windowbox Supply and specialist topiary companies