I used strips of felt to keep the stand elevated so I can get airflow underneath to dry any potential spills.
Are you are trying to protect the floor from being damaged by the stand itself of from potential water spills? My stand is on carpet but to protect the carpet from spills I bought one of those blue plastic tarps from Home Depot. Then I cut it about 8" larger than the stand itself and stapled it to the inside of the stand. Any water that spills from the sump, reactors, etc. is caught by the plastic tarp.
I would not put the liner between the cabinet and the floor, sooner or later you are going to have a spill and water will get between the liner and the floor with no way to evaporate. I like posted above use small leveling feet (felt or neoprene) that lift the stand up 1/2" off the floor. If you have a spill you can use a towel or paper towel to wick up most of the water.
how thick is the felt you use?

Fexible pvc shower pan liner is sold at Lowes and HD. It makes a waterproof 'tub' in the bottom of the stand. I also installed a bulkhead at the end and ran a pvc pipe through the wall and out of the house. It has saved me a couple of times with small leaks, like an external pump that leaked. It also makes working in the sump and refugium much easier as I don't have to worry about drips and minor splashing when I clean the skimmer, pull filter socks or harvest Chaeto.
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I have a grey waterproof mat I bought at lowes. It looks like a puzzle piece. you can make it the size you need. I lined the inside of the stand with it also. Water just sits on top until you dry it up and its also kills any vibrations and noise. great stuff.
http://www.lowes.com/pd_295099-47189-MB06014_0__?productId=3111467
Looks like a winner. Thanks for the help!