Mirrored tissue box covers may come without a frame. Usually, the crafter can just glue mirrors together, and it supports itself. However, this kind of setup is a lot weaker. The glue can weaken over time, and the entire thing could break down. You really don't want to break a mirror as the shards can be a real hazard.
Hence, it's best that you buy a mirrored tissue box cover with a frame. This allows for a stronger configuration. The frame could be in cardboard, wood or metal. Cardboard is the weakest, and metal is the strongest. Wood is in the middle.
It's best that you buy the most durable frame material you can afford. Not only will the mirrored tissue box cover last longer, but it will also avoid injuries and cuts caused by broken shards of glass.