i have the voltphreaks vph900. i weighed it, and it is exactly 7lbs as they state. works perfectly. starts the car strong, even after it sits for three weeks.
i had previously tried the antigravity rs-20 (10 lbs. / ~$400) car battery. it did not work well in this car. the battery goes into a sleep mode, and the car acts like it is not attached to a power source as a result. it was registered correctly, there was just intermittent starting with it because of the connectivity problem.
back to the vph900- i love it. cost me $1k because i picked it up with cash, and they threw in SAE terminal adapters (don't forget to ask for these no matter what you choose).
i used two 2"X3" adhesive backed velcro pads and secured the battery to the battery tray using that. i then used a velcro strap and fed it under and between the pinch welds of the oem battery tray and up and over the battery. sometimes, the gods of speed just make things easy on you. it was perfect. below is a link to a similar strap.
http://www.homedepot.com/p/VELCRO-Br...0441/100096349
the 2"x3" velcro pads were rated at 15 lbs each. the battery is definitely not going anywhere with the pads and strap, and there is no metal to puncture the battery in the event of a collision. might not pass tech, so check your rules and use at your own risk. another secondary benefit of the pads is vibration reduction through the sort of padding that velcro provides.
i am very happy with this setup and can post a few pictures if you would like.