Many koi-keepers do have Flocor or similar material in their filters. This material should be cleaned out every 6 to 8 weeks in the summer when the fish are feeding, and it can be quite a chore removing this material from a large filter bay.

The netting used is pond netting from garden centre's such as B&Q. This is one metre wide. The netting is cut twice the length required for the bag, and is then folded over until it is double.
The netting is then sown together using nylon, and it has been found that the best material for this is the reels sold for strimmers. One side is left open until the Flocor has been added and then the whole side is sown up.

This photo shows another view with the flocor being laid out so that the bag is almost flat.
The bags can be put into the filter bay this way, or as in the view above depending on the size of the bags in relation to the size of the filter bay.

This photo shows a close-up of the stitching. This is not very important, and can be stitched through the holes in the netting at about one inch intervals.
Several members have been using these types of bags for five or six years and have found them very useful, and they have not given any problems.

Due to the dimensions of the filter bays or any pipework restrictions it may be difficult to cover the whole area with the nets mentioned above. In this case the bags shown in this photo can be used.
These were purchased from a koi dealer, and are said to be safe for use in the filters. The blue tape at the neck is already threaded through and so after the Flocor has been added the tape can be pulled tight and tied.
Some koi-keepers do use old onion bags, but this is not advisable as you do not know what material they are made of, and they could have been sprayed with some insecticide or worse and this could still be present.