I'm so sorry to hear about your carpet - I would be so upset and what a ridiculous practice!I had a long tentacled anemone (macrodactylis I think??) for about 9/10 years in a standard 6 foot 125 gal. Was pretty good size, foot was about 4 inches diameter and stretched up a good 8/9 inches probably. I ended up making a small acrylic box about 6/7 inches square, filling it about 90% with fine grain reef sand, then set the foot right on top of the sand. It eventually dug down to the bottom within a day. Lived there for many years till I sold it. Was cool cuz I could grab the box and move it if I needed to do some kind of tank maintenance etc... I kept it in a corner against the back and the over flow box and placed live rock in front to hide the container so it looked more natural. Not sure as to the carpet anemone, I would size up the animals current dimensions and tailor the box to fit.
Side story: I purchased said anemone along with a fluorescent pink carpet anemone when I first got into this hobby (1996). Typical noob, rookie story. Inexperienced. Bought animals I thought I had done enough research on but hadn’t. The carpet died 1 to 2 weeks later. Figured out later it had been injected with dye to make it more colorful. I placed the long tentacle under the brighter portion of the tank with compact fluorescents and it would “walk” over the span of a day or so to the opposite end of the tank which was only lit by regular fluorescents. I’d move it and it’d “walk” back over to the “dark side”. After the 3rd or 4th time, I figured it was going where it wanted to be. Plus I was tired of peeling the foot off the rock which is not the easiest.
Sorry for the long post
The box idea is super neat - unfortunately my sand has been placed and so far so good. Just have my eye on some large wrasses now to hopefully dive in and stir up the sand