The larger the planter boxes the better, but you can get by with smaller dimensions but the plant will get rootbound quicker.

I would recommend a clumper, as it will do much better in a planter. If you choose a variety that is too 'thick' or grows too tightly together, you can simply take out some of the culms to give a more open appearance.

I don't know of any small(12 foot) varieties that grow completely upright. You can top culms to any height you like and they will remain at that height, that is, if you choose a larger variety.

What is you lowest temps in Winter?

There are a number of Bambusas that will do well, although they lean out a bit.

Here are some Bambusas to choose from...ABS

B. glaucophylla aka MALAY DWARF is a beauty if you like variegated leaves. It does grow as more of a bush and isn't very cold hardy.

Others, are some of the B. multiplexes.

Kt

Chusquea culeou or similar. On the general market here, so unless suppliers down there aren't also providing garden centers with it you should be able to find it there.

I am growing Himalayacalamus 'Damarapa', the candy stripe bamboo, in a large container. Does very well in the container for me, and I think it would make an outstanding screen.

I also am growing Himalayacalamus asper. I think it would also would make an outstanding screen in a container. It's a bit more upright than 'Damarapa'.

I also grow Drepanostachyum khasianum in containers, which is more open at the base, but the canopy is wide, and I don't think it is suitable as a screen, though it can be kept upright by shortening the culms.

All are clumping bamboos.

P.S. I have way more of the 'Damarapa' than I need/want. If you want some, I'd be happy to sell half or all of mine. I'm located in the Bay Area.