You can use a sponge, sphagnum moss, vermiculite. Like Allee said, raising the humidity to such a high percentage isn't good for a leopard gecko.
Why did the vet say to raise it that much? If it is a skin problem i.e. retained shed, there are other ways to remove the shed and prevent it from happening again.
Allee has many leo's in her care, and has rescued some too. She know's what she is on about. She is only looking out for your gecko's health, and trying to prevent illness in the future. My gecko has an upper respiratory infection at the moment, and she is on the anti biotics. It isn't very nice having to put them through something like that, even thought the infection wasn't caused by something I did, it is still heart breaking to see her ill.