I don't know what your sundeck looks like, or how much strength you have. Just remember, individual containers are fairly easy to move around when needed. Once you construct a large box, unless you put it on wheels, it will be impossible to move. Also, aren't you afraid of rotting your deck, with a large box in contact with it?

My worry with several boxes is the weight on the balcony. I have not planted much yet...because I want boxes or a wine barrel or something.. but being on a second floor and the balcony feels or sounds funny when I walk on it with just my hevy weight I worry it could hold anything heavy.. any suggestions... The balcony floor is cement.. but the apartment has shifted and foundation seems cracked..slightly..

If your balcony is bouncing or creaking slightly when you walk on it, then I wouldn't put things that are too heavy out there. You could use those large containers but fill them halfway with some type of fill material - foam peanuts, empty soda cans or bottles, etc., then top off with soil.

I guess I didn't give it too much thought, greenthumb, seeing that I live on the second floor, moving it would be impossible. Not to mention, didn't give any thought at all to the possibility of rot. Guess I'll stick with the containers up there, still would like to build a box, maybe for the front yard...