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I'm interested in building some inexpensive planters for our driveway.
I want them to be about 12' long, 2' wide and 2' tall, with open
bottoms for drainage. They don't HAVE to be square but that's probably
the easiest shape to work with?

I make 2'x8'x(1'high) planters out of regular 2x4s and 1/4" plywood. I'm
building 3 new ones for Spring planting. Just build a 2x4 (or rip the
2x4s into 2x2s) frame for the bottom with (in my case) 1' risers every
two feet. The risers at the corners don't need much support but in the
mid span I connect the two sides about 6" up to prevent bowing. Screw
the frame together, cut the plywood to fit the sides and screw that to
the frame. On the bottom piece drill 1" holes for drainage.

I then prime the entire box with a good primer like Kilz and then use oil
based paint that they use for porches on everything. For 12' long you'll
have to piece together the sides since plywood comes in 4'x8' sheets.

It's not necessarily cheap, wood is getting expensive nowadays. If I
were to cost out each of my boxes it would be:

1 1/2 sheets of plywood at $15/sheet = ~$22
5 8' 2x4s @ $3/each = $15
1/4 gal Kilz = ~$3
1/3 gal oil paint = ~$8

For a total of about $50. Again, not that cheap but it's a big box and
no stores sell planters this big. I estimate it takes me perhaps a total
of 6 to 8 hours of labor to assemble and paint each box. This year I'm
building 2 that will cantilever off a roof which will require a little
more lumber and engineering to figure out loads making sure it doesn't
collapse.

So my plywood box survived all of last year and this winter unscathed and
looks as good as when I installed it with no degradation to the plywood.
I think the oil based paint had something to do with it. I was thinking
of experimenting with exterior latex paint since oil paint is a real PITA
to deal with when cleaning brushes and it gives off toxic fumes making it
difficult to paint indoors.

To see this version of the box that I made go to:

http://www.brandylion.com/images/front_box.jpg

Note: I forgot to add that since the box is suspended, there are 2' 2x4
"joist" beams screwed underneath below the bottom every 2 feet to keep
the plywood bottom from sagging.

I'll be building my new boxes later this month so maybe I'll put up a
photo diary as to how they get put together.