I have an acclimation box, but I needed a second one for my frag tank, since I had to acclimate new fish to both display and frag tank at same time. Thought of using my spare gutter guard and zip ties. Also had spare acrylic to use as a top, but you could use a dinner plate since most people have those. Don’t really like using egg crate, plus it’s less flexible to work with. For clownfish anemone acclimation, people used an acrylic slider tube, but that is a one shot slide and hard to find different sizes, plus probably don’t keep it in the tank overnight, no water flow through it. You could make this any diameter as your nem rock (if it’s on an island) and seal the fish in. Also could do the same with Filefish or Copperband sealed on an aptasia rock. Also could make it horizontal to allow for more swim space, just use more gutter guard to cover the ends. Then use a mag cleaner to cling it to the tank. With bare bottom, just had to be a little careful to keep it sealed at the bottom it might float up when adding fish, or just make a bottom cover. Sand probably works better cause you could dig it in.