I use these and believe me they are worth it. 
They can hold cels up to 11x14 in size. The mats 
are seperate from the backing, so they are not 
like clamshell mats. The way it works is that you 
put the cel in a cel bag and cut some holes for 
air to escape. Then you tape the top two corners 
of the bagged cel into the mat. Then you place 
the matted cel into this clear sleeve that is 
open on both sides. I keep the matted cels in 
those clamshell boxes that are sold on the same 
page as the mats. Each box will hold about 15 
cels. I have found that this is the best way to 
keep your cels flat and free of dust. The setup 
is not airtight, which is good. But it will keep 
the cel from getting dirty. Plus, you can disply 
the cels with their backgrounds without the two 
actually touching eachother. And you can place 
and sketches or genga in the back of the sleeve. 
Hope this helps!