Have you already planted your veggies or flowers? If you haven't just dig a trench and then drill a hole in the side of the box to insert your soaker hose into. Run the hose though the bed and and out a hole in the other side and then cover it back up. If you have already planted and you don't want to disturb the plants, you may try taking taking a piece of pipe big enough to insert the soaker hose into and using a sledge hammer to drive the pipe through the length of the bed. Insert the soaker hose into the pipe and then pull the pipe out through the other side. Now the soaker hose is on the bottom of the box. Of course this would be very hard to do if your boxes were longer than four feet but I think if they were only four foot beds, you could probably accomplish it this way.