I just got some fabric at the place where I got the polyfil. Then I got the staple gun out and stapled down the edges of the fabric and stuffed it with the polyfil.
depends what kind polyfill you are gonna be using.If you are using the loose kind then you can staple a piece of wire mesh inside of the box where the port begins, if it's the "roll" type (wal-mart) then thats easier , just staple it with a wood staple gun.