I was having a space problem in my main tank last year and needed some temporary space to put corals that I was fragging or frags that I had gotten from the swaps.
Here is how I have made moveable shelves that stick to the inside of the aquarium glass. These have worked out so good that I have to be careful that they don't become permanent fixtures in the tank! If you are using frag plugs, this shelf works perfect with them.
1-Cleaner magnet (get a cheap one that is strong enough to stick to the thickness of your tank glass) Cost: ~$10 from Petsmart
1-chunk of florescent lighting grid (comes in 4' X 8'sheet)
1-piece of thin plastic (1/16" to 1/8" is thick enough) EDIT: if it's too thick, you will have to get a more powerful magnet!
Glue for plastic - (something that will actually melt the plastic like PVC glue)
Saw to cut the plastic and grid. I use wire cutters to cut the grid.
Cut the grid to size....3" X 9" is a good sized shelf
Next remove the scouring pad from one of the two magnets...this is the magnet that will be in the tank water. Scrape it clean and flat as you want it to be sealed when glued to the plastic back. Do not remove the pad from the other magnet that will be on the outside of the tank.
Cut the plastic backing to the width of the shelf grid and the magnet width + 1/4". For instance: The shelf width is 9" wide. The magnet is 1.5" X 3" in size. The plastic backing is cut to 1.75" X 9".
I got a little fancy and cut this piece with some contours:
Next, glue the edge of the grid onto the longest edge of the plastic backing at a 90 degree angle (can use clothespins as clamps):
After the grid glueing had dried, glue the magnet under the grid (clamp down while drying so that there is a good glue seal:
Here is it attached to the tank...almost looks like it is floating. Just be careful when cleaning the outside glass....if you knock the outside magnet off, the shelf will fall to the bottom in slow motion!
After a couple months, coralline will start growing on the grid and cover up the white plastic: