You can buy sewing sheets and measure them. cut them to the size you want and take the sides off office/report binders and slide them on the sides of the sheets. This will keep them in place.
I bought some sheets at 90cents/sheet at walmart.
The office report binders are like a buck for a 2 pack. I bought in bulk and it cost me 10 dollars for 50.
Only other thing you need is a measuring tape or ruler and some scissors.
pic of the sheet cut down to fit a 10 gallon
Pic of the report binder sidings and sheets
Slide them on the sheet sides...
And presto! We are ready to place them into tank after a rinse. (I'm paranoid I rinse everything).