you can use a green plastic fruit basket with narrow slots,the kind you buy strawberrys in , where the grass will poke through but be held down by the plastic grid. you should be able to cut with scissors a nice piece out of the bottom or 1 of the 4 sides.Sometimes mesh onion bags are usefull with other plants. Dark ,someone on here had a great idea I kinda forget ,something about filling a filter bag (or onion mesh plastic green bag)with some gravel then placeing the grass on it then tieing the top or sewing it with mono filament,allowing the blades to poke through the mesh,they said you could easily take it out to prune it. NOTE food packaging is supposed to be inert so rinsing it should be safe for the aquarium depending what was in it .Like old moldy produce
