On 120/240 service panels I try to panel mount the surge protector in the box. NEVER come across the unbreakered side and I suggest installing two 20 amp breakers side by side in your panel in you have the room. Those two breakers (one on each leg) are only for attachment to the surge arrestor. The value of those breakers may be slightly different (15-25 A), depending upon the continual current rating of the protective device. Like has been said, you do not want a house fire. Also quite a few arrestors fail in a shorted mode instead of an open mode an that can be "a very bad thing". On a transient event you may be tripping those breakers with some regularity. You can install a 240 volt light bulb (some sort of night light for that area) so you will know right away if the breaker is tripped and you can reset your surge protection. If the breaker trips immediately then you know you have a protector that has failed in a shorted mode.
There are lots of possible panel-mounted protectors out there. Here is what I use;http://www.lashen.com/vendors/leviton/spd-51120-ptc.asp
Some others may be like these; http://www.elecsupport.com/products/surge-protection-devices/medium-exposure/powersure-panel-lpl-series/https://docs.google.com/viewer?url=http%3A%2F%2Fact-communications.com%2Fpdfs%2F452S-Cutsheet.pdf
(I like this one as it also has an all mode filter built in, use a 25A breaker even though it says it is good for 30A.)
Since I take 2-3 direct lightning strikes on my house a year (just the roof, not including the antenna farm or the trees in the yard) I also installed 20 Leviton hospital grade surge protector/ harmonic filter/ GFCI trip wal outlets throughout the house. (the orange colored front panels, they also have a warning light and a buzzer that goes off in the breaker if the protection fails). Anywhere I have electronics is installed it is plugged into one of those outlets. Quite a few of the breakers in my Siemens distribution panel have also been replaced with GFCI breakers. It is not unusual on a lightning strike to have half a dozen GFCI devices trip out all over the house, GFCI breakers to trip and for the big 25A circuit panel breakers for the house surge protector open up. One time a direct strike on the utility pole in my yard even opened up the 200 Amp service entrance breaker.
Alabama Power was too "cheap" to drive in a ground rod. When they hit solid rock three feet down they cut the rod off and attached their ground wire to it. I have since dug up a fifteen foot long trench, two feet wide, two feet down, starting at their "cheap" ground rod and following along the drip line of where the rain comes off of my roof. I laid in a giant coil of copper mesh grating that is 15 feet long and nailed it to the ground with four 2' copper rods (until those rods hit rock too), silver soldered the rods to the grating and silver soldered that to the original Alabama Power ground rod. Then I flooded the trench and dumped in ten pounds of magnesium sulfate (Epsom Salts) and backfilled the trench while still mucky and stamped the entire thing down into a mud-pit. I left their ground wire in place but I added a 2" copper ground strap from the mesh, up along the back of the house and removed the lid to my electric meter and silver soldered the copper strap to the lip of the meter box. I painted the strap white so the copper thieves do not think that it is a shiny new toy.