Find some rocks or buy them from the LFS
. I have some pretty pink shaded sandstone. I used two pieces, one about 3 inches deep and 6" long, the second was about 4" X 4 x4. On top of them I laid a large piece about 12" X 6" That makes a good size cave. I also leaned another piece at an angle. I have the base for that against the glass so it won't slip and fall on top of my eel.
I have the large rock about 1 1/2" away from the glass so the eel can crawl behind it and on the side and hide if he wants. I have plants around it as well. Sometimes he buries himself in the sand within the plants.
You could also get a pvc
pipe and lay pieces of slate or stone to hide it. I have a foot long pipe in the big tank that you can't see, but the fish use it either to hide or to get from one end of the tank to the other.