I needed a fresh coconut for some recipe so I bought one at the grocery store. I just cut the coconut in half used what I needed and scraped out the rest. I boiled the crap out of both halves of the shell and then used a dremel to cut the doorways and drilled a hole on the opposite side-near the top-of each shell-half. I did this so the water would flow through and so I could peek into the shells. Don't forget to sand the edges of the coconut. I use the other half for my plecos in another tank. They love it too!
Crays are strongly benthic. If you want some critters that will use the upper parts of that big tank, why not add some large shrimps, such as Atya, Atyopsis, or Macrobrachium species?