You'd need some kind of electronic logic chips to properly convert the signals
Actually, all the needed logic can be found in the parallel port controller and the pad itself. Some diodes are needed for power, but that's nothing too serious.
and a driver for the PC to get it all working.
Which can be found
here.
I doubt you'd be able to do it so cheaply yourself.
I don't. Still, the PC Magic Box is what I use myself, as it's a whole lot less trouble in general (in particular it doesn't require finding an extension cable or cannibalizing a dead Saturn to avoid permanent conversion of the controller). So far my only gripes with it are:
- It doesn't quite init fast enough, but it turns out that there's a way to compensate for this in Linux (apparently it's a somewhat common problem among HID devices), so it works fine for me now. I assume Windows has something similar or MS would be knee-deep in complaints from hardware manufacturers.
- The button mapping flat-out sucks. It seems to be based on the PSX layout, which doesn't translate real well; the order is something like: Y, B, A, X, Z, L, C, R. Of course, any sane app will let you remap the button functions to deal with this.
- Only the basic controller protocols are supported on the Dreamcast and Saturn interfaces. This means that the analog stick on the Saturn 3D pad doesn't work, and C/Z on the Dreamcast arcade stick don't work, among other things. This is fine if you just want to plug in a basic Saturn pad, but it's a little disappointing.