The universal O2 sensors will work just as well as the ones specifically for the Boxster. They are identical sensors that just don't have the plug on the end. The only problem would be if your plug is damaged.
How to wire it properly:
The wire needs to be soldered and use a heat shrink jacket to cover it and make it water tight. Whatever you do, do not twist the wires together side-by-side or use crimping to connect the wires. The sensor actually "breathes" and cools itself through the gaps between the wire and the insulation so you don't want to block that with crimping or a huge knot of wire. I haven't used them on the Boxster yet, but they should be the same as the 928 with three wires, one black and two white. You mate the black to black on the plug and the white to white. It doesn't matter which white each white wire on the sensor is mated to, they're interchangeable.
All that said, if you can get the OEM type Porschev found for $10 more, I'd definitely do that instead of having to wire in the universal.