I would not be able to do this. It would drive me crazy. lol. But I have spent a lot of time dreaming of a display refugium above my DT. I decided if I did it I would place a small sized MJ pump or eheim inside the corner overflow of my DT to the fuge.Ya I’ve been thinking about putting a pump in dt. Over time coraline will overgrow and make it blend in.
So what are the goals you hope to accomplish with adding a refugium? Are you wanting to add water volume, do you have a mandarin you are hoping to feed? or have algae for nurient export? Or all of the above. I can understand ditching the cannister filter. Many people get misdirected in their use when they are still new to this.
Also are you capable of drilling a smaller aquarium? I think this would be the best route to go. I had a refugium above my sump and it worked very well. I also once set up a small hospital tank above the sump and had some interesting challenges.