Drainage layer ideas?

So I'm building an enclosure for my ball python, and I'll be making it bioactive. We've ordered supplies online, and a few came today, including two bags of hydroballs.
The issue is that it won't be enough to give a good layer spreading across the entire enclosure.
Now I know the drainage layer isn't a necessity unless the plants inside need quite a bit of water. I've selected species that are drought-tolerant, and thrive in pretty much any environment, since I don't want there to much too much moisture inside for my ball python.
In that case, should I worry about the amount of hydroballs inside? If so, what else can I use that I can readily find (we're kind of on a budget, and there aren't many suppliers here; it took forever trying to find one)? I know gravel is used, but it also isn't that great?
Do I only need a thin layer?
Thanks in advance:)

I once had a tank (not bio active) with Hydra Balls on the bottom layer. It worked fantastic in humidity control.

Eco earth was on top. I would poor water directly on the Eco Earth, even large amounts. All of the water would drain to the bottom and be gently released through the top layer as humidity. The top layer was never wet or mushy.

I never had humidity problems with that system.