The hard pipes at the front of the car are a different diameter on the S compared to the base model. What this means is that the new "T" pipes that are used to plumb in the center radiator are too large for the pipes on the Base.
Pelican have a DIY where they use standard brass plumbing as a reducer to allow the correct size rubber hose to attach to the hard pipe, but I have seen first hand this fail at the track (after 2 years of being installed) causing coolant to spill out, annoying all his mates at the track and ending his day early. I don't recommend this method.
I personally just tightened the heck out of the hose clamps using the standard pipes, and have had no issues after 2000 track miles.
There used to be someone who welded the S ends on the base hard pipes to make the best solution, but the name escapes me. This would be the best solution, but additional cost. EDIT - It's precision chassis works.
http://www.precisionchassisworks.com/boxster-cooling-line-modification.html
Hope this helps.
PS. if you are not tracking the car, there really is no need for the center radiator. If you are overheating in normal traffic, there is another fault you would be better tracking down than trying to mask the problem by fitting the center radiator.