Step 1: Supplies
1 cheap side table, I used the Lack side table from Ikea - $7.99
4 sheets of poster board, I got 2/$1.00 at Dollar Tree - $2.00
Adhesive of some sort, I used mounting tape but I'm sure you could use regular tape or hot glue or whatever
Step 2: Measure, Measure, Cut
I decided to take off a half inch on the bottom because the table will be sitting on carpet.
Using a pencil on the wrong or inside part of the poster board I mark a square 21.5"(W) x 17.5"(H). To compensate for the 1/4" difference between the top and bottom I make a mark at 2" down on the height and a 1/4" in. This should give you a T shape.
After cutting out the first piece and checking it against the table I use it as a template on the other 3 pieces of poster board.
Step 3: Attach the Poster Board
After attaching the top, add a piece of tape to the bottom corners and smooth it down the sides to make sure it is flat. Repeat with 2 more sides.
Step 4: Add the Cat Door
After you've cut out the door just attach it like the rest, except maybe add more tape because this is where it might fall apart from your kitty rubbing against it while entering and exiting.
Step 5: All Done!
I thought about using foam core, but I know my cat will tear into that in no time. If I were looking for a more permanent solution I might use some bead board and me it look really nice.
These tables come in lots of colors, or maybe you or your kids could decorate the poster board for fun.
Please note that I had to measure the space under the table (18") and find a litter box that would fit between the legs. I found one at a big pet store, but it's really only big enough for one cat. If you have multiple cats this might not work for you. Or if you have a larger litter box already you might need to buy a smaller one.