I found a nylon snap in led holder at radio shack that fits well.
part number 276-079
It has a lip around it that stops it from going completely into the cart so it works a little differently, but you can put a variety of materials in this and it makes a nice sturdy removable plug.

Right now I am using it with two layers of stainless pipe screen. I poked a small hole through the screens with a nail to allow a little better flow and easy refilling with a syringe tip.