It is a very substantial, large cabinet, fastened either in the center of the High Altar or sometimes to one side of the altar on a pedestal. Usually adorned with gold leaf, lapis, etc.
This is our Tabernacle:
If somebody steals a tabernacle, they have to rip it off the altar or the pedestal. That usually requires large tools, etc.
I would probably cheer if Our Lord surprised them with what happened to Uzzah when he touched the Ark. But then I would have to go to Confession . . .