Watering potted plants is more efficient if they have trays under them, but the thirsty ones run out faster than the others. Putting them all in the same "tray" means they can share.
In our case, this project also helps hide the untidiness under the deck...
*We're also using our shower and bath water to irrigate a "compost biomass" garden (comfrey, sugarcane, culinary ginger, etc).
Step 1: Materials and tools
polythene (the thicker the better)
duct tape (gaffer, duck, etc)
hose fittings (see picture)
drill (preferably electric)
craft knife (although a decent pair of scissors would be better)
Step 2: Prepare deck and build frame
The deck need not be sanded smooth, but it is important to check for unseated nails, splinters* and rough spots. Naturally you should do this before building the frame, but hey.
*Cheapo deck timber, but has lasted over 20 years.
Step 3: Prepare plastic liner
As my plastic used to be a painting dropcloth and had holes, I had to check it against the sunlight and stick bits of duct tape on the holes on the side that would not show.
Step 4: Lay plastic in trough and check for lowest end
Fold the plastic in half lengthwise and lay it in the trough. Make the front edge sit about 1cm below the top of the front board and put in a staple 1cm below the edge of the plastic, about 30cm from one end. Put in two or three more staples along that edge, but keep away from the ends (you will need room for cutting and folding).
Have a discussion with your gorgeous helper about which end is the lowest. Even though you're sure she's not right, loosely fit the plastic in place and pour some water in it. Gracefully admit you were mistaken, and lift the plastic to drain out the water. Use the dog's bone-eating-towel to dry the plastic - this towel will also be useful to kneel on*. This whole bit could be avoided if you use a spirit level beforehand.
* Three uses for the same towel. Points for the first one to make the obvious towel reference: extra points for the actual quote and book.
Step 5: Drill the drainage hole
If you have two layers of plastic, carefully fold back the top layer, and stick on a 200-250mm length of duct tape folded back on itself so the sticky side is out. Press it on to the lower plastic layer above where the drainage hole is to go.
Carefully roll back the top layer and press it firmly onto the duct tape.
Stick two more short lengths of tape on the top plastic layer above the previous tape. This is all to make a firm base for the drainage fittings.
Use the holesaw guide bit to drill through all the tape and plastic layers into the deck. Do not let the saw teeth touch the duct tape.
Fold the plastic back and drill through the deck. Be cautious as you near the end of the cut as you could suddenly break through the deck and almost smack your head on the upper deck...
Step 6: Fit the drainage fittings
Place a backing board behind the plastic and use the craft knife or scissors to cut a hole about 3mm smaller all around than the line you just marked.
Carefully force the hole over the top nut (so that the nut is poking through from the topside). If you are concerned about leakage, you may want to run a bead of hot glue or silicone sealer between the top nut and the upper duct tape before you screw everything together.
Firmly screw on the bottom nut and place the assembly into the deck hole.
Gently screw in the riser assembly. The riser can be made from anything with a thread that fits the top nut - in my case two adaptors gave me the approximately 50mm I needed. My top nut already had a rubber washer, but you will need to find one if your's doesn't. Or use thread tape.
Step 7: Finish folding and stapling the plastic
The corners can be tricky, and you may need to cut off some excess*. Remember, though, that the plastic will not be readily seen once the pots are in.
Remove any staples you may have used to temporarily hold the rear edge of the plastic.
* "Measure twice, cut once"!
Step 8: Place pots in trough and water them
Put your tools away and tidy the workspace.
This trough allowed for eight pots with room to spare for wider spacing of the plants that need it - not planned, just serendipitous*. Taller plants, like the Bog Sage near the steps, would hide the under-deck mess better, but the parsley, sage and thyme** are easy to get at for cooking with fresh herbs.
* "The Three Princes of Serendip" http://en.wikipedia.org/wiki/The_Three_Princes_of_Serendip
** "Scarborough Fair" http://en.wikipedia.org/wiki/Scarborough_Fair_(ballad)