There wouldn't be anything harmful in the velcro, but I'm not sure it would be worth the hassle. You'd have to silicone at least one of the sides to the glass anyway so you lose the movability--unless you're thinking of just swapping items in and out.

If you're looking for branchy, I'd head out after a big wind storm and see what was down that would be useful/suitable and then prep that to use. Or check Black Jungle or a bunch of other places to see what they have.

The little clay hydroballs are one of the staples for drainage layer/false bottoms used. If you look through the terrarium build area you'll see a bunch of that and the egg crate stuff.