Here is a concept I have been working on for some time. The basics are this:
The green represents the caves for the craws
The red is the holes through which the craws can actually crawl through to visit other parts of the tank
Each section is divided using a solid colored plexi. Plexi is used so that it can have a couple dozen 1/2 inch holes drilled in it for circulation. The bottom of the plexi is up to 2 inches off the bottom to allow for circulation and substrate. This can substitute for the connecting tunnels as the craws will undoubtedly burrow their way under it anyway. The top is also below the waterline by as much as 3 inches for circulation.
2 powerheads are set up kitty corner from each other flowing toward the other end.
The actual dimensions were based on a small crawdad I found online, I think it was a bumblebee craw as they only get 2-3 inches long. My original dimensions were 48x24 because of their small size. Larger craws need larger tanks off course and you can modify the number of sections. I had 5 sections because they craws are supposed to non aggressive and it would have housed up to 10 of them. Only being 14 inches tall to allow for ease of maintenance and moving caves and stuff around as well as removal of food and gravel vacs. Each section has an individual cover so they can be opened individually and it would be easier to secure the lids so the craws can't escape.
I would have put this in the DIY section but I thought it would was better suited here. That and it is just an idea because I have not actually done this, yet.