I have a Husky trunk. I wouldn’t use a cover on it because, as others have said, it’s just one more thing to clean and move around. I did make an attachable pad for the top so I can sit on it to pull on boots or have a quick snack between rides. For that I took a lawn chair pad, sewed velcro to it and then stuck velcro on top of the trunk so it’s secure. My show pony has a tendency to grab and fling things she can pick up so I had to “Cupid proof” the pad.
In terms of decorating, I primarily put stickers on the inside of the cover and just have my monogram on the outside to mark the trunk as mine when there’s several of the same in the barn aisle. I do have a friend who uses a trunk cover for eventing and I love how it looks, but she spends a lot of extra time cleaning it each season and taking it on and off during a show.