I don't know if this is the right thread for this but I just figured this out and thought that others with an IM Fusion 20 might be interested.
I've struggled to have a large enough ATO tank to handle the evaporation from my tank. I found that the Fluval Chi (5 gal.) tank fits perfectly (if you take the door off to get it in) into the IM 20 stand. It doubled my holding capacity and I'm really happy about that.
...just in case others are struggling with the same issue.