I agree...I believe paper towel is best as well and necessary for the juveniles.

I just wanted to add..........
-be sue it remains FLAT
-be sure there is enough water to hold the corner/edges down
-you don't want a frog to get under there

FYI -- I have never had substrate in my red eye tree frog tree frog enclosures-
heavily planted --but no substrate. ( my boys are 3-5 years old )