Life gets so much easier (and cheaper) when you do this:
1.) get some 3/4 inch pvc pipe.
2.) cut (4) half inch pieces
3.) hot glue those half inch pieces to the bottom inside of your 6qt iris tub.
4.) drop your light diffuser on top of its new pvc pipe legs
5.) NO SUBSTRATE! simply put water in the tub until the surface of the water is just under the diffuser.
6.) put top on, and leave alone until you see a snake head popping out.
It is that simple. No substrate needed.... No plastic wrap / press and seal / or anything like that needed. My motto is quit playing in the sand and trying to guess if your humidity is right, and just make it right.