You could grow them as coppices or pollards. However, if they ever got away from you...even if hard-pruned annually they will still get some size to the trunks and roots, probably making removal a headache.

What about putting up a trellis or fence?

That is just about the worst choice for a tree if you want to remove it later. Poplars are a "pioneer" tree that send fresh stems up out of the ground from their roots.

If it's definitely a temporary screen you're after, why would you not simply erect a fence of the appropriate height behind the arbs? That seems much simpler (unless I'm missing something) than planting something that's going to grow larger than you want, and needing to prune it all the time.

A fence would have been my first choice all along. However, this area sits to the side and in front of my house. In order to block out this boat/trailer I would need an 8-10 ft high fence. Unfortunatley there's an ordinance which will only allow us to build a 4 foot high fence. This won't do much of anything. The city doesn't control what you plant so that seems to be my only option. Do you have any other ideas for trees./shrubs?

I would love to know what you decided to do, as it’s been over a decade since your post! I’m interested in growing hybrid poplars in a big livestock trough, to keep the roots in check and get fast screening in a spot I can’t plant (all concrete backyard).
Was planning to prune them back to 5-6 feet or so every fall, then let them grow in the summer...
any thoughts on this?