Personally I would go get some the interlocking floor mats that you see used around weight equipment, and that way it gets rid of two issues. First is not having to deal with the ugly floor. Second is that it will cut down on the sound from the equipment and still give you easy access to the backflow preventor in that box.
http://www.amazon.com/CAP-Barbell-I...1453973042&sr=8-5&keywords=rubber+floor+tread
The other would use what we did in front of our washer and dryer downstairs. We got a huge roll of left over Rubber tread mat from my wife's work. You can use this and just lay it down. If you get a backup or have water issues, you can roll it up and take it outside to hose off and then dry. http://www.amazon.com/gp/product/B0...rd_t=36701&pf_rd_p=2079475242&pf_rd_i=desktop
http://www.amazon.com/CAP-Barbell-I...1453973042&sr=8-5&keywords=rubber+floor+tread
The other would use what we did in front of our washer and dryer downstairs. We got a huge roll of left over Rubber tread mat from my wife's work. You can use this and just lay it down. If you get a backup or have water issues, you can roll it up and take it outside to hose off and then dry. http://www.amazon.com/gp/product/B0...rd_t=36701&pf_rd_p=2079475242&pf_rd_i=desktop