I've used nothing in mine and I love it that way!!!!

Wow 30 bags!! That's amazing!!!

I went looking for the foam core at Joann's but couldn't find it, but found 11X14 thin pieces of canvas on sale so I will cut it in half and make a sleeve. Hope it works.

I used template plastic, and rounded the corners so it wouldn't poke through the fabric.

I use the fiber board from Dollar Tree.

At the dollar tree I found the plactic cutting boards. You get 2 pieces for 1.00. I get a total of 6 pieces to make 3 bags. I also make a sleeve for them.
Next time I am going to look for the foam core.

Platic canvas

plexi glass with the corners rounded