This is a cheap way to provide a water reservoir for pots all shapes and sizes. I had already a summer flowering climbing plant in large metallic container but this was very deep and therefore inefficient. It was easy enough to pull out and replant. The idea is very simple and is adaptable to the materials you have.
Step 1: The Cutting Part
I had a spare length of perforated surface water drainage pipe (yellow) and some leftover waste water pipe (black) from a bathroom job. I also had two green cut flower buckets although something smaller would definitely work.
I used a hacksaw and pair of tin snips but you could use a knife or similar item to do the next bit. Cut a hole in the yellow pipe to snugly fit the diameter of the black pipe and cut slices out of the end of the black pipe. Finally cut a hole into each bucket to fit the yellow pipe, for me it was the size of a cartoon mouse hole.
Step 2: The Assembly Part
Now to bring everything together in location. Depending on the cuts in each of your pipes, you can arrange all the bits in place to be in the best place for your materials. Mine fitted well centrally on the bottom given the size of the planter and its height. You may wish to cut your pots differently so your pipe is at one side or to the rear.
You can use plant pots with holes in them already but if your pots like mine don't have holes on what was their bottoms, make sure you drill/punch a few holes to allow the air to escape when filling up.
You can add a few pebbles to the bottom or just fill with compost/soil and plant up.