I have been taking a 3/4" oak cabinet grade plywood sheet, cutting it to compliment the footprint of my stand (i.e. My new 120 gal
. is 2'x4', so I cut the plywood 3'x5'), and placed a raised edge around the entire perimeter of the sheet using 1.5"x 1/8" pine edging strips. I then stain it to match my stand, and coat with several coats of polyurethane. Finally, I seal the inside edges with silicone sealer. Then I place it on the carpet under the stand.
It will only hold about 5 gal
or so, but it works for small emergencies. I haven't figured out how to handle major spills yet.
If you move the aquarium in 5 years of so, it seems like the carpet springs back quicker as well.