Plug for watering system in window box

I have window boxes for shrubs and succulents on my windows. To ensure that water reaches the roots in summer, I placed a plastic pipe in the soil (I assembled the pipes into a T-shape so that they covered the entire bottom of the box and drilled holes every 5 cm). However, the pipe outlet causes drying, so it is a good idea to put a plug in place to prevent the water from drying out. The plug has a diameter of 30 mm. It fits into a 32 mm pipe and is held lightly on a rubber seal so that it can be easily removed by hand.