You can fit a MOMO shift knob to the Boxster shifter but it will require you to round out the top of the shaft to fit, not a difficult thing to do with a drimmel and grinder bit attached.
Yes it will be quite obvious, the MOMO knob is round and the stick of the box is rectangular so you will just round off the four corners a bit at a time till it fits.
Took me maybe 10-15 minutes with a dremel, make sure to wear gloves as it will get hot, also I did not do this while in the car, I had removed it as I was installing a short shift kit at the same time.