I know, I love weeping moss! Right now I'm considering redoing the plastic mesh though, I think the moss might not be getting enough light. I'm going to see if there is some kind of glue I can use.
I based my under gravel filter off of Mordalphus' design, seen in this post: Setting up a tank (a how to, sorta)
As you can see, there is a system of tubes with holes underneath the substrate, I made my holes facing inward, then I covered the tubes with bio-media so that the substrate doesn't get sucked in, and to provide a lot of surface area for a nice bacterial colonization.
The difference is that I use an HOB filter to power the under-gravel filter. However, I don't think this is powerful enough, and also I haven't figured out how to connect the tubes. I might switch to a TOM Mini Rapids canister filter. This way I can also create an inline CO2 reactor for better CO2 dissolution.
I used a UGF because since water is being constantly passed through the substrate, you can grow a great big bacterial colony for nutrient stability. It also acts as a kind of prefilter for the shrimps so they don't get sucked in and made into filter sushi.
Since this is a nano shrimp tank, this works well because I don't need a tone of water circulation.