Hi
This is probably of no interest to anyone else but...
When I bought my Mindstorms (for US$15 in a charity shop) it was missing the IR Tower.
I'd allotted myself an hour to build a Tower but I suspect the IR components I was using were the wrong wavelength and I couldn't get it 100% reliable. So I just fitted a miniature jack socket into the RCX and have a long lead my the PC. Not as convenient but the price is right.
The transmitter and receiver components are at the front of the RCX:
Unsolder them and put them somewhere safe.
Now solder a stereo coax cable to the pads. I used one from an old pair of earphones:
Any small signal transistors will do. I used a BC639 and BC640 but they're overkill for a job like this. The transistors invert the signals and advust the voltage levels. The 100n on the signal from the RCX smooths out the 30kHz signal which is (normally) modulating the IR signal.
You can use either the 9-pin (female) or 25-pin (female) connector depending on what your PC wants.