I put florecent light grating on mine to hold the media down. Some other folks put the media in mesh laundry bags so there is no need to put something on top. It's all preference.
I have two skippies, one I cut small holes in the grating and planted perennials directly in the grating with the roots hanging down into the media, no pots. In the winter I will just remove the grating and set it in the pond while the skippy is turned off. I planted some mint, watercress, hosta, monkey flower, and sweet flag so far in this one.
The other skippy will just be filled with Water Hyacinths floating over the grating as soon as it warms up enough to buy them.
I cut a large circle of the floss stuff used for padding, and used a second fluorescent grate to hold it in place over the top of my media. This keeps the laundry bag strings from trying to float out my barrel outlets, but the floss also helps trap floating algae and finer particles from the water, and with it on top, I can remove and clean the floss in a few minutes. I might have to throw in some hyacinths this year.
My hyacinths were a bust last year... I don't know if it was because of bad plants, the new pond, or the fish getting too aggressive at eating the roots, but last year was the first time my hyacinths didn't completely take over. I figure if I start them out in the filters this year, it will tell me if the fish were the culprits.
This year, I purchases a mesh laundry bag and put my filter media inside it. I then tied the strings to the tub. Last summer, my DH made our version of s Skippy Filter out of a tub eh bought @ Walmart. I believe it is the type they use on farms w rope handles on each side. He made the aparatus for inside and assembled it while I was out of town. I had already gathered those nylon scrubbies and scrubbing pads. Occasionally, during heavy rain, the tub would overflow and filter media would escape so when I found the nylon mesh laundry bag, we decided to try it. So far, so good! We really need something bigger though, but he is currently working out-of-town so we will have to do with what we have. Unfortunately, the Green Frog does not like the filter system so he moved to the other side of the pond. I am sure he is w8ing for floating plants and floating pots to settle onto.