Hi there, you can put a box like this right in with her. It's better to keep the coco fiber contained so it holds it's shape + moisture. I just cut a portion out of the top and keep a log next to it for access. The box is about 22 X 13 X 8. Filled at least 3/4 or a bit more.
Yeah, you definitely do not want to do that, and coco fiber isn't ideal either. The reason it needs to be dampened is because they need to be able to dig a tunnel/hole that will sustain it's integrity so that she can lay them in it and bury her eggs. So it needs to be in a big pile/hill that she can dig a tunnel in. Sand dampened is better, or at least mix some in with the coco fiber and then dampen it. But no, do not just put coco fiber throughout her tank, she needs a proper laybox with the substrate dampened, as AHBD is instructing.