This is what I use for all the seed I collect. Paper allows seeds to breathe, and there is far less chance for mold should some seed not be as dry as you thought.
You can start with any size square. When I am making a bunch, I find it easy to grab a stack of printer paper, the common 8 1/2" by 11".
Cut like so:
Then you end up with two of these squares:
Fold in half like so:
Then fold one side in like this:
Then the other side:
A small piece of tape on that flap will hold it together, then fold the top over like this:
And when you are ready to fill it:
Easy as can be. When filled, fold the top flap back down and tape shut. Even small seeds like poppy don't escape.