I use the tray watering system for my carnivorous plants, but had lots of trays and pots all over the place. It was tricky to get the amount of water in every tray right, and the rate of evaporation was different, so it required frequent monitoring. Also, larger trays gave better humidity around the plants than smaller trays.

I was at the lumber store and spotted some free wooden pallets, and hit on the idea of using a pallet to make a tray for my carnivorous plants. With some pond liner to seal it, I made a nice wide tray with a consistent depth, humidity, and easy maintenance, and it looks tidy too.

Materials:
A wood pallet, free from lumber store
Saw
Hammer
Pond liner from pet shop. See step 5 for size.
Extra boards (optional)
Extra nails (optional)

Step 1: Take the Boards and Nails From the Pallet

We'll only be using half of the pallet in this tutorial, so start by taking off all the boards you don't need. Try to keep the slats intact, because we'll use them later on to make the bottom of the tray. Keep the nails you pull out as well, unless you have some that you plan to use.

I pulled off all the slats except for two, which will be the start of the tray.

Step 2: Saw the Pallet in Half

Pretty self-explanatory - cut off half of the pallet. I picked the nicest half to keep, and cut off the more broken side. I kept the 2x4 in the middle so that the tray would have extra stability if I had to move it around.

Step 3: Put Boards on the Side and Bottom of Your Tray

Use the nails and boards that you have from step 1, and put 4 boards on the bottom of your tray.
If you don't have enough boards or nails you'll just have to go out and buy some. I used a couple of slats to make sides for the tray.

Step 4: Finishing the Bottom of the Box

Use a few lengths 1x1 board to tie the bottom of the box together. This also keeps the box a little off the ground to prevent rot. None of this needs to be pretty because it's all going to disappear in the next step.

Step 5: Line the Tray With Pond Liner

This holds the water.

The tray liner you get should be large enough to wrap around all the sides. Since the tray is six inches tall, an extra foot and a half all the way around is good. Don't forget the 2x4 in the center of the tray is going to use of a foot of length.

The liner came from a pet store and was about $1 per square foot.

Tuck it in, and make neat folds in the corners.
If you want the corners to be perfect, leaving the liner in the sun for a while or heating it with a hair dryer will make it easier to mold to the shape of the tray.

You can glue the liner if you want, but gravity, the water, and the plants will hold everything in place.

Step 6: Fill the Trays With Water and Put Your Plants Inside

Fill the trays with water and put your plants inside. Duckweed on the top of the water reflects heat and helps prevent evaporation, and looks nice too. I've tucked the extra pond liner under the tray to keep the wind from lifting it, and it looks tidy that way.