The favorite way of using this kind of trough feeder as a planter is to build in a reservoir to hold a supply of water before putting in the potting soil. This gives you a planter which will require less water and fewer waterings.
Tape over any holes in the bottom of the trough and then drill a series of 3/4 inch diameter drain holes in the walls of the trough about 1 1/2 inches above the bottom as shown here. When the water depth gets over 1 1/4 inches deep, the excess water will drain out through the drain holes. I put drain holes only on one side of the planter so that water would drain toward the outside of the deck and not all over the deck floor.
If you want to prevent your soil from running out the drain holes, use a sheet of landscape cloth to line the trough. If you don't have access to landscape cloth, sheets of cardboard will keep the soil from running out with the water.
Put a generous inch of gravel in the bottom of the trough to act as a reservoir. Even layers of newspaper will do. If you don't have access to gravel, fill the reservoir space with Perlite or Vermiculate. Anything that is inert and non-water absorbent will do. Marbles, broken pottery, even bits of brick can be used.
Once your reservoir space is filled to the bottom of the drain holes, cover the filler with landscape cloth or sheets of corrugated cardboard. Acually anything inert that allows water to percolate through will be fine.
Make up a potting soil of 1/3 Perlite or vermiculate, 1//3 composted manure, and 1/3 topsoil and mix it thoroughly. The final mixture should be light and easy to work with.
Fill the trough with the potting soil and cut off any surplus landscape cloth or cardboard and you are ready to plant
This potting soil is about 1/2 as heavy as an equivalent amount of top soil and your planter will be light enough to move if needed.
The runners on the bottom make it quite easy to rearrange the planter on a deck or on a patio.